David,

The frame I have is a full plate frame for glass plates but I have window glass in it for film/ paper negatives - so I would be looking for something larger than this for paper or film (if I get around to glass I'd be using the one I have I reckon) - so probably 10 x 8 would be the smallest size that would interest me - the idea of a frame that's quite large (but not too unwieldy) with inserts sounds good. Sorry if that's not specific enough. I need to do a bit more to know exactly what would be best - but I do think you'd find interest as I didn't find a lot of choice when I started looking.
Cate