Medium SPRs used size A bearings. The small SPR bearing was the same as the small YYJ bearing, which was sometimes called size B. It was used the Aquarius, Sunset Trajectories, and a handful of older 1A yoyos like the Super Spinfaktor. I don’t think it is used much anymore except for in YYJ loopers.
The outer diameter is only slightly smaller than the medium SPR bearings, but the inner and outer races are very noticeably closer together, which is the easiest way to tell them apart (the inner diameter of the size B bearings was actually the same as for size C). The small SPR bearings were also narrower than the medium, so the gap would be a bit smaller.
After HSPIN and SPYY started using size D bearings, most people started referring to D as medium and A as small for everything except SPRs, so it got to be a bit confusing when medium SPRs still used what most people considered small bearings.