A full-size keyboard is about 440 mm wide. Most of that width is the number pad, and most people never touch it. Cutting it gives the mouse somewhere to go, which is the real reason the layout matters: your shoulder, not your desk.
TKL: everything except the number pad
Tenkeyless keeps the function row, the arrow keys and the navigation cluster. Nothing moves and nothing needs a modifier key. Coming off a full-size board, a TKL feels identical minus the part you were not using.
The cost is width. A TKL is around 360 mm, so you save roughly 80 mm.
65%: arrows stay, function row goes
A 65% drops the function row and the navigation block but keeps the arrow keys. It lands near 290 mm, about 150 mm narrower than full size.
The trade is real. F5, Delete and Home move onto a layer you reach with a modifier. For most people that is two days of annoyance and then nothing. For anyone who lives in spreadsheets or renames files all day, it never stops being annoying.
How to decide in one question
Look at your right hand on the mouse. If your elbow is pushed out away from your body, you want the width back and a 65% is worth the adjustment. If your desk is already wide enough, take the TKL and skip the learning curve.
One thing outweighs both: if you use F-keys without looking, buy TKL. You will not train that back.
What we stock
The 65% board is 290 x 102 x 40 mm with 63 keys, and connects three ways - cable, 2.4 GHz wireless and Bluetooth - so it moves between a laptop and a desktop without being unplugged.
The 87-key board is the TKL: wired, with hot-swap sockets so the switches come out by hand.