Basically, creatures should be the same size (but L++ is pixel-doubled from Lemmings, so there's more room to work); flavor them however, but slightly cute is preferable.
For terrain, unless you want a challenge, you probably won't want to follow the old masks from the original pieces. As long as terrain sets have pieces that can make a wide variety of layouts, anything should be fine. If you can make something in the same style as the original graphics, all the better. :)
Answers are at http://lplusplus.co.cc/graphrep.php
Basically, creatures should be the same size (but L++ is pixel-doubled from Lemmings, so there's more room to work); flavor them however, but slightly cute is preferable.
For terrain, unless you want a challenge, you probably won't want to follow the old masks from the original pieces. As long as terrain sets have pieces that can make a wide variety of layouts, anything should be fine. If you can make something in the same style as the original graphics, all the better. :)