Have you got a car in mind already? You say new to Robin Hood so assume buying either a completed car or a unfinished project? Do you know what model of Robin Hood it is, if a monocoque (series 3, s7 etc) there are some bits to be aware of for IVA, but nothing that can't be sorted fairly simply. Lightweights should be avoided, 2B's are simplest to get through IVA.
However if you are buying new then it will be a gbs zero, in which case I think dimensions are on the gbs site.