By right-clicking the marker and pressing PageUp, you can increase the set height of your character (in increments smaller than layers) at the beginning of the level. However, because of physics, the player will simply fall back to the ground and not float, provided that there isn't something else for the player to fall on. If you want to create a walkable, invisible surface, download the segment editor and make a texture with a fully blacked-out Alpha channel--that way, you'll just walk on "thin air".
Evil has a new name. Demo out now!