Here are some photos of the tranponders in a few of my cars. They illustrate the size of the transponder, 1.25" square, in its current, through-hole form. The first shows an HPI 235mm-wide HPI 1/10 scale pan car that I use in our local Breakout class. Plenty of room here:



Here's a 200mm-wide SpeedMerchant WGT car, still with plenty of space:



I had to mount the transponder on top of the steering servo on my XRay T1 VTA car (the smaller AMB transponder was on the chassis directly on front of the receiver):



When we get to a 1/12 scale car, this Carpet Knife 3.2, things start to look ridiculous, but it does barely fit, and still makes minimum weight: