A Dilemma?

Yesterday I started implementing the level into the game, and I knew something was going wrong right away. My screen resolution for the game was set to 640x480, so each "screen" or block on the map would be that size. But it was much to big for what I had planned. So I experimented with that size and it didn't fit. Everything was to big, so I changed the screen resolution to 480x360 which is a very weird screen size, but fits the game perfectly. Hopefully this doesn't affect (effect?) the game when I decide to port it to the Xbox. I've roughly placed out the collision blocks for the first 14 or so rooms of the game totaling about 30 screens and a weird glitch surfaced. When you fall in some weird way (maybe when you fall in the line between two adjacent blocks if that makes any sense) you go through the ground. That is definitely a game breaking glitch that I need to fix. Oh well. Lets hope for the best.