If you don’t need pixel perfect accuracy, you can use the ItemSize option. Based on your example, this should work pretty well, since all cells look to be the same size. Read the documentation for ItemSize. You’ll see that you can set horizontal and vertical size. Probably easiest to just play with some values until you get the size you want.
If you want pixel perfect accuracy, you’ll probably be better off using the various Image manipulation functions. Turn your strings into images, pad them to get the dimensions you want for each cell, and then ImageAssemble them.