There are 2 Lidar programs. one creates a grid under Grid|Grid Create and one that creates a Triangle Mesh under Utilities|3D. It sounds like you're doing the first. The question is how many cells there are in your grid. Its a common mistake to set grid dimensions that are too large. While your Lidar data has a very fine resolution, using that resolution will end up with a grid surface that is unwieldy. While there is no fixed limit to the size of a grid it would be good idea to limit the grid size to 300 x 300. You can adjust from there.
Let us know what dimensions you're using.