If all you have is a 'sprout', you have a long time to go before worrying about leaf size. The leaves are growing to soak up more sun and help the tree grow all around, and now is not the time to be cutting them. Ditto re twig growth really. Next (early) spring think about which major branches you want to keep or not, with an eye to the future (don't lop off what looks wrong then until you're sure it won't fit with your 'final' design). If things get very bushy and messy now you can shape twiggy growth to look neat, and that alone will encourage more twigginess, but you have to let it get more established altogether before doing a lot of hacking.

Grow a http://www.evergreengardenworks.com/trunks.htm TRUNK
Then worry about small leaves.
See additional link below

Here is a link that might be useful: Pinching elms