You'll get suggestions.
Lots of lenses with interesting history. Some more interesting than others. Pick one and ask around here on RFF.
The Canon 50/1.8 can be an outstanding lens. It will be a bit larger than the I-61 (which I assume you have based on the f/2.8). Others will agree; pay attention to possible haze in the Canon 50/1.8, its a known issue and you definitely don't want it. It sometimes can't be cleaned off. Most other lenses with haze can be successfully cleaned.
Other 50mm lenses that are smaller than I-61 are mostly going to be collapsible. I don't know about collapsing lenses on an RD-1 so definitely get guidance from others here who have the RD-1. In general though, I don't think you'll find a 50mm lens that's not collapsible and appreciably shorter than the I-61. Its just the nature of the lens design. Now, if you want to use a shorter focal length, there are many options of wide angle lenses with shorter physical dimensions.
Just some of the possibilities in 50mm-ish that are fairly compact:
1. Leica Elmar 50/3.5 (collapsible)
2. Industar-22 50/3.5 (collapsible)
3. First version summicron 50/2 (collapsible)
4. Leica summitar 50/2 (collapsible)
5. Cosina-Voigtlander Heliar 50/3.5 (collapsible)
6. Cosina-Voigtlander Color Skopar 50/2.5
7. Canon 50/1.9 (collapsible)
8. Canon 50/3.5 (collapsible)
9. Minolta Super Rokkor 45/2.8
10. Leica Summarit 50/2.5 (recent model)
11. Jupiter 8 50/2 (no more compact than the I-61)
12. Fed 50 or I-10 (collapsible, not much different, if at all, from the I-22)
13. Leica Summar 50/2 (collapsible)
14. Many others. There are many more 50's out there but most are not collapsible and some are substantially larger than the I-61. Also, price varies quite a bit. You can pay as little as $20 for some of the FSU options, or upwards of $10,000 for a Nokton.
You might also want to specify a budget range to help narrow the suggestions you'll receive here.
