Recently announced Canon RF 28mm F2.8 STM lens is impressively compact for an f2.8 wideangle lens with Full frame sensor coverage. Here is a scaled size comparison of the new 28mm with four other most compact Full Frame lenses in RF mount:
(Click on the image for the larger version)
The Canon RF 28mm F2.8 STM is clearly the most compact lens for the mount. Below is a specs and feature comparison of these five lenses:
Canon RF 28mm F2.8 STM vs Canon RF 24mm F1.8 IS vs Canon RF 24-50mm F4.5-6.3 vs Canon RF 50mm F1.8 STM vs Canon RF 16mm F2.8 STM Specs and Feature Comparison