I am a newbie to adult mountain biking, and recently bought an inexpensive adult bike with 26" tires. The size below that were the 24's, which were designated as for "youth" sizes. However, I cannot stand with the bike without leaning over considerably to reach the ground on one side, and feel like I'm jerking when I'm peddling, because I have to nearly straighten my legs to get a full cycle. I also think I might be sitting up a little too straight on it. I'm going to exchange it; what should I aim for? Keep directions simple, please!