Still researching, but the L.12 that you have shown is the Entwurf III version, which never made it past the windtunnel due to high drag (and incapability of ever going supersonic) and was redesigned into the Entwurf IV version where the pilot was prone (laying down) so the intake didn't have to divert around the cockpit. So, the plane as designed would have never been armed. Most info out there is about the last version, (and most models of the L.12 are that version too). The best I've seen so far on the III version is int he following image:
You could take an average male height and extrapolate the length of the plane from the sideview, then extend to the rest of the plane... Probably going to give you the best idea of the size/scale.
As the engine, it looks like the design was based on a radio-controlled bomb GB 3/L, so if you can find info about that, you might have what you need. I see one site offering dossiers on various luft46 projects, but unfortunately they're not free. Reichdreams Dossiers