Should you decide to purchase a Waxwing bag its important to consider how it will mount to your bike. There are different solutions for different types of bags; here's what you need to know:
Handlebar Bag: Randoneuring style handlebar bags have a roomy box shape and need to be supported from underneath-- typically by a small lightweight rack. There are a variety of rack options available from Velo Orange, Rivendell, Giles Berthoud, Compass Bicycles, Boulder Bicycles, and others. My handlebar bags come with a backstop sleeve that securely affixes around the backstop of the rack--this give the bag a firm grip. An additional feature is to include velcro straps mounted at the base of the bag to secure the bag down to the rack. Those two elements together make the bag nearly inseparable from the rack, although the bag may sway a bit when full. Typically this type of handlebar bag employs a decaleur as well, which is a non-load bearing stablizer to keep the bag from swinging from side to side. In some instances the decaleur is built into the supporting rack as a single unit, in other cases it is mounted to the bike stem. Decaleurs have the advantage of acting as a quick-release, easily allowing you to take your bag with you.
As far as size guidance, typically the decaleur bracket that mounts on the bag will land somewhere within the top 1 1/4" of the bag so it lands on the horizontal leather band. There's nothing written it stone about this, but it's what happens most of the time, looks right, and would only be complicated by an unusually tall bag. The distance from your front rack platform up into the zone of your bar height it going to define the likely space for your bag. For bag shape the variable that tends to change the most is the height--the footprint doesn't seem to vary too much--ranging in the 5"-7" in depth and maybe 10"-11" in width. You don't want to crowd your brake hoods; you can come close dont let the bag complicate your reach; you don't want to have a bag that looks mushed in between the hoods.
If you should decided to create a cardboard mockup, don't think too much about the pockets that fix to the bag--getting the box shape is whats important. Sometimes people have particular requirements (fitting a laptop, iPad, or tire pump for example) that becomes a factor in working out the bag size, so add that into your considerations.
Starting points for reference:
5.5" d x 8" h x 10.5" w for a small 5.5" d x 9" h x 10.5 w for a medium 6" d x 10" h x 11" w for a large
I've made bags as short as 6" and as tall as 13". It all depends on the needs of the rider and the space afforded by the bike and rack arrangement. Early on I had a bag that came even or maybe even a little higher than the bar height and I found it was excellent for blocking the wind on my fingers, but a bag that high requires a decaleur that can accommodate the height.
Your handlebar bag comes without any holes in the top leather band on the back of the bag, but will need them to mount the decaleur, if you plan to use one. Depending on the type of decaleur you have, install the part (the receiver) that attaches to your stem first. Then, to determine where holes will be needed on your bag, place your bag on your rack and affix the backstop sleeve and bottom Velcro straps (if you have them). With the bag settled in place you can then place the decaleur bracket that will mount to the bag in the receiver and locate where you'll need to makes holes for the mounting bolts with a marker. Make sure your marks are even left-to-right and top-to-bottom before you make your holes.
To make the holes, the ideal tool is a rotary hole punch, but a tubular hole punch from a grommet set is fine too. As long as you are careful, the hole will be covered up by the decaleur bracket, so you have a little leeway in terms of neatness. On the inside of the bag I like to mount an interior maple backer, but fender washers will work fine too.
To stiffen the bag, I use a piece of coroplast cut to the right size fill the bag running from under the interior cover flaps down and across the bag. It helps to ease the corners so as not to abrade the fabric. No physical attachment is necessary. I throw a $20 bill in the bottom of the bag under the stiffener for some just-in-case cash.
Saddlebag, Cape Roll & Tool Pouches Saddlebags are pretty easy at attach to your bike; the two straps that come out of the top of the bag simply slide through the two metal loops at the rear of your saddle. Some saddles don't have these loops, in which case you have to come up with something else, like a different saddle or small attachable loops that fix to the saddle rails. Same deal for Tool Pouches and Cape Rolls. With smaller saddlebags, its not unusual to use a single strap to mount to the saddle: what you do is start from inside the bag, come out, go through the saddle loop, come back in the same hole in the bag, pass out the second hole, go through the other saddle loop and then go back into the bag through the second hole to complete the sequence. Attach the strap to itself and it's done. This keeps the attachment looking neat on the outside and makes the bag a little more secure since its not obvious how the bag is fixed to the saddle.
Panniers: Panniers are typically designed as either front or back mounted bags. Because the rider's heel moves through part of the space a rear pannier inhabits, its pretty standard to either have a narrow bag or have a corner of a wider bag "cut off" to allow the heel to pass without contact with the bag. Front bags can be roomier since they are not limited in the way that rear bags are. Like a handlebar bag, pannier need a rack to mount on. Old school pannier mounts are basic but effective j-hooks that hook over the rack tube with a bungee that hooks low to keep the bag in place. There are more elaborate mounts that make attachment much easier and more secure, but cost more.