personally I reckon it sould be a little wider - so the car could be parked along side it, and room for a lift/jack-guy in front and behind (F1 style) - ie to me the pit building should be the length of a long car + 2 people and a tiny bit extra.
Take a look at the "Bit Char-G tracks Developing Kit" page. The buildings are intended for Micro R/C but as that's about the same size as Micro Scalextric they shouldn't be too far from the size you want.
They might be useful for reference even if you go with your own designs.
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