I see it posted time and time again to our discussion forums about how to make a bike fit both shorter and taller riders alike. So, I figured I'd see if I couldn't condense the discussion here.
Areas to consider in no particular order:
The aftermarket offers complete seats that are both shorter (-1/2") and taller (3/4-1"+) than stock. Both are affordable and bolt in in minutes.
Another route for shorter riders is to cut down your stock seat foam. Mark on the seat with a Sharpie pen the material you need to remove, make your cuts with an electric knife and ideally, fine tune the seating surface with an air grinder and medium grit sanding disc. But, don't cut too much off, otherwise you'll be sitting on the top of the bike's frame (not comfy).
This is why complete, bolt on shorter seats are a better choice, as the density of the foam has been adjusted accordingly.
Some foot pegs incorporate mounting bracket collars that allow the peg to be mounted either up and forward or down and backwards. Most stock foot pegs leave a bit to be desired in my opinion, so not only can adjustable foot pegs better your riding position, most are substantially wider than stock, offering the rider added comfort and control.
Suspension (shorter riders)
There are a couple of options here. The first are lowering links. These aftermarket units replace some or your complete suspension linkage and can drop the rear of the bike as little as 3/4" to over 2". Most are bolt, so they are easily removed should you grow or sell the bike to a taller rider. You can also slide the fork legs up in the bike's triple clamps when adding a lowering link. How far? I'd recommend consulting with the lowering link manufacturer to see what is working best for their customers.
You can also have your suspension professionally lowered. I've had this done to a bike in the past and it can be done without hurting suspension action. While the bike I had lowered spent most of its time off-road, I did play on the MX track and the bottoming resistance was excellent. I suppose it just depends upon the skills of the suspension tech you choose. I used John Curea of MX-Tech East Coast Suspension.
Handlebars and Mounts
Handlebars come in all sorts of bends. Bends refer to key measurements used for comparing the different handlebar choices. These measurements are width, height, rise, clamp area and and sweep:
Generally speaking, taller riders will want bars with more height and less sweep (aka pullback) and short riders will want the opposite. The best way to select a new handlebar is to first record the key measurements of your existing handlebars to establish a baseline. Then, sit on your bike and pull your hands in the position that feels most comfortable to you, noting their position in relation to your existing handlebars.
Selecting bars isn't an exact science, so don't worry if new handlebars feel a little strange at first. With a little seat time, you'll get used to them.
If you're selecting handlebars to mount in your existing bar clamps, the key measurements of the handlebars is all you need to consider ergonomically. However, if for example, you're also changing the bar size from a stock 7/8" a 1 1/8" (aka fatbar), you'll also need to factor in any additional difference in height the new handlebar mounts may add.
A popular choice are 7/8" to 1 1/8" bar mounts/adaptors/risers. Most are going to add an 3/4" in height, so you need to consider the additional height of both the new handlebars and mounts to arrive at the total new height. Also, some handlebar bar mounts/adaptors/risers are offset, meaning that the handlebar does not mount dead center in the mount. A typical offset is 5mm. And some of these bar mounts can be run in the forward position (+5) and lipped around towards the rider (-5). So this offset must be factor in with the sweep (pullback) of your new handlebars to arrive to the final sweep position.
Lastly, there are also triple clamps (top clamps) that allow the bar mounts to be moved farther or close to the rider. Some use sets of holes (E.g KTM OEM) and others use v-shaped, horizontal channels allowing for even more fine tuning of the fore and aft position of the handlebars relative to the rider.
Clutch and Brake Levers
Short and tall riders may also have proportionately small and large hands. Usually a complain more from riders with small hands are clutch and brake levers where the throw is too long, making it difficult to easily use these critical controls. Some bikes come stock with levers that have adjustable through, but for those that don't, the aftermarket often has you covered. If not, forged (not cast) levers can be bent to suit the reach of the rider's hand easily enough.
Tall rider + big feet = tough time shifting. Once again, the aftermarket has these riders covered with extended shift levers that allow the rider to get their boot under the lever. Some levers are similar the stock, but an inch longer. Others are a bit more sophisticated with replaceable shift lever heads in -10, 0 (stock), +10 and +15mm.