Tekin RS is about 30mm x 30mm x 10mm high (plus a little bit more height for post/wire joints), the RS pro being a bit higher with its extra bank of fets. I run my RS in my GT12 car, with 13.5, but its perfect in my 2wd buggy with a 10.5 also. They will run to 7.5 I think limit, so fine for your vintage car I'd have thought.
I've not observed anything smaller, but that's not to say something smaller doesn't exist?
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Team Yokomo - YZ-2CAL3 | YZ-2DTM3 | YZ-4SF2 | YZ-2T | YZ-2DTM Worlds | YZ-2DTM x2 | YRX-12 x2
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10electric | Batley Buggy Club | YORCC | RHR | MB Models
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