you could give the smaller man a good starting position like half guard or even mount then work on getting back to lock down and your butterfly hooks in, because you will burn alot of calories and be awesome at escapes after.
Congratulations on your first class! I'm a smaller guy(even smaller when I first started) so I felt most comfortable with closed guard and maybe a little butterfly guard. Since then I've went through phases where I would try one guard for a while then I would see a certain set up or sweep from another and try it for a while. I expect you'll do the same. I would say don't limit yourself, don't get too comfortable with one position and work on what you or your instructor feels you need to work on most. Have fun doing it!