Size has nothing to do with ability.
Take Kessel as a prime example. He doesn't go in the corners, he doesn't, fight, he doesn't 'outmuscle' anyone. But, he has a scoring touch and a way with placing that puck in the net with such accuracy that this alone lends validity to "size has nothing to do with ability".
Even during the more violent NHL era of the past, incredibly talented small players such as Marcel Dionne who toiled for the L.A. Kings, survived using their skills and hockey know-how. Dionne was listed at 5'6".
The problem with the Leafs is not that they lack the size to outmuscle teams. While true that our forwards seem 'soft', it doesn't mean that they can't get the job done. It's the system they used that either didn't fit the players or vice versa.
I believe that Carlyle would prefer a team of bigger, bruising forwards and defenseman. Perhaps a player or two, an Evander Kane type would help to prop up one of the forward lines, and a defenseman who can actually play without the puck would help. But the Leafs don't need wholesale changes to their roster en masse. They've got a good core group and should stick with it while upgrading and at the same time, develop the drafted players who should be up and coming.