no prob, the key to entering is your footwork, once you get your footwokr down better, you'll notice you start dominating the bigger guys with poor footwork. I've been fighting pro and semi pro Muay Thai and MMA since 1999 and my instructor still smacks me around and he's 5'8" and 140lbs. Soemtiems it feels liek I'm a little kid trying to fight his dad, for every strike I land he lands 4 or 5, all due to his superior footwork. He gets in on me when he wants in, and is always that extra inch otu of range when I try to land on him.
If you can, get some UFC tapes and watch Randy Coutore fight. He does an excellant job of getting into the clinch on aggressive strikers, his match against Vitor Belfort was a textbook use of lateral movement and footwork to close the distance and destroy the attacker.