Here's a photo of an NCE D13SR decoder before and after addition of the S-CAB radio receiver. By placing the antenna cross-wise we get a smaller package. With receiver and decoder mounted back-to-back, the result is thicker (0.32"), but length (1.5") and width (0.75") are not much bigger than the decoder.
Mounting the antenna in this manner is not recommended by the radio manufacturer. However, tests show good radio reception (at least 20 foot range), which is adequate for loco control. In future, this configuration will be available with no change in list price.
I plan to test this same antenna arrangement with Tsunami TSU-1000 and TSU-750 sound decoders. Because of its small size and simpler wiring, the 750 is a particularly good candidate for a minimum sized sound package.
I'm a retired electrical engineer, but still spending more time on engineering than on my layout. These days, it's mostly about applying radio control and battery power on smaller scale layouts (HO, On3, On30)
The photo above is not my layout. It's a great view of Seattle's King Street station by Ross Fotheringham.