There’s a quad stretch where you lay one leg out long then cross your other leg over it at the knee, then twist your body. Because of my size and shape, however, I can’t fully cross my leg and have my foot flat on the floor. Is there some kind of modification to this stretch that I can do?

The important thing is that you can feel the stretch. Don't push too hard. It's okay that your foot won't touch the floor yet. If you are trying to stretch the outside of your thigh, try crossing your leg over and hugging it to your chest instead. If you are trying to stretch your back muscles you can sit or stand and softly pull your arm across your body with your other arm or stand with it against a wall corner and turn gently. Stretch enough that it is mildly uncomfortable but never do it to the point that you are hurting. Btw, it's good to do a little warm up before you stretch (jumping jacks, dancing, short jog). Think of your muscles like silly putty. They stretch best when they are warm.