The short answer is probably not. The next step down for a widescreen 16:9 LCD from 26" tends to be 17" and I've not seen many that have the necessary connections for HD. There are some 20" around but these tend to be standard 4:3 models which don't count.
To be honest going below 26" would be a waste of time for HD anyway as you wouldn't really see much difference.
Apologies, there are some 23" models around from Philips and JVC that might be OK.