Snap to grid is in the editor, under the view tab.
For 45x45 blocks, you would want 15x15 or 5x5 for a grid size. I don't think there is really a "standard" size for textures, but most people seem to use powers of 2 (2, 4, 8, 16, 32, 64, 128, etc), probably because they provide the largest number of multiples and are most easily divided into smaller pieces.
Also, what Ashley said. <img src="smileys/smiley4.gif" border="0" align="middle" />