The only guideline that I would give would be to make sure you have enough room for 8' lanes if it's asphalt. Anything less and you start to get pinched.
It's actually FUN to watch novices (and pros
) on narrow lanes - I usually end up being a source of amusement too - but it's better racing with at least 8 feet. ROAR requires 10' for asphalt.
Basically, if you keep to 8' lanes, you could get by with about 35' x 40 since you really need at least 4 lane-widths wide and at least 5 in length (if you have only four, it gets a really boring track).
But, use what you got! I've seen smaller! With carpet the widths can be smaller.