Largish means fairly big — not huge, not small, but on the large side of ordinary. The -ish on the end is a softener. English uses it when the speaker does not want to be exact: smallish, oldish, greenish. So largish is a hedge. A largish room is bigger than you expected, but you are not calling it large and being done with it. It is informal, everyday speech, more spoken than written. You would not put it in a science report; 'a largish sample' sounds sloppy there. Use it when size is a feeling, not a number.