Not sure what you mean by enlarge cells. If you mean combine cells, so that one cell takes up the space of other adjacent cells, you can't do it.
If you mean something else, please describe what you are trying to do in more detail, and maybe I can help. No guarantee.
Updated: I was wrong. The newest version of Numbers does allow the merging of cells. Somehow I missed that in the update, along with some other nice cell formats. Sorry.