Sizeable means fairly large — big enough to notice, but not a word you would use for something huge. A sizeable crowd, a sizeable chunk of money, a sizeable difference. It is a comfortable, slightly soft adjective: you are saying “this is no small thing” without giving a number. British spelling prefers sizeable (with the e); American spelling usually drops it and writes sizable. Both are the same word. You will hear it in everyday talk and in writing; it is not slang and not stiff. It does not mean “can be measured” or “able to be given a size”, even though -able often means “can be…”. Here it just means “of a good size”. Compared with large, sizeable often feels a little milder, a little more spoken.