AMT Star Trek 3 ship set

The ships are, well, tiny. If I am thinking of the same kit as you, the AMT 1/2500th Enterprise A, B, and original series ships, the largest (the Enterprise B) is about three inches wide, seven to eight inches long. Takes up very little space, but beware bent struts (that may need to be re-aligned to avoid listing nacelles) and misfits that will leave gaps.

Klik

The Romulan ship is about 3 inches long and 5 inches wide. The Klingon and Enterprise are about 6 inches long and 4 inches wide. They come on a base that is a triangle 6 inches per side. Each one mounts to an arm that is about 5 inches long.

i think 2500 scale is the only constant scale for all the enterprises

I'd go with the 1/1000 kits.They are not very big.

Small is my aim - I normally model 1/72 - shelf space is a premium but I'd like to have a crack at having a Enterprise set

tos are pretty small i think they were about 1000ft long? so about 4-5"?