Just to clarify, because I feel your last sentence is a little ambiguous. The Peco box vans in L&B or Southern livery are accurate in height, width and length according to dimensions published in 'The Lynton & Barnstaple Railway Measured and Drawn' by Stephen Philips. The only real compromise is in gauge, to enable them to run on OO9 track.
OO gauge figures on the other hand, in my experience, tend to be often 'over scale'