I generally think small is better in many things, but I don't think I would consider it a make or break point in your case. If your kids are very introverted or really seem to have a hard time making friends, I'd probably lean towards the smaller school system. If they seem to have a strong talent or interest in something that one school is stronger in, I'd lean towards that school (like a much better choral program for a child who adores singing, or science/computer resources for a child who adores that). But I don't think the size of the middle or high school would be critically iimportant to me otherwise.
As for class size, our public school has small class sizes, usually 15-20 per class, so a class of 24 or more would make me ask extra questions. I'd especially ask if the larger class sizes are indicative of budget cuts and difficulty getting enough money for a good educational environment. But if that wasn't the case and I felt like the teachers and school could handle the larger class sizes, my concerns would ease.
Advice and commentary on living overseas
DD1 15, DD2 12, and DS 9