To answer this question, you must know how would like to print your mosaic.

If you work in a pre-press bureau - you may already know this information and may not need read further.

I can give you a few simple tips:

1.  If you want to display your mosaic on a monitor (or Web site) only:
The size of your mosaic in pixels must be no more than the typical computer screen resolution:
800x600, 1024x768, 1280x1024 or maybe a little more. The majority of Web surfers use 800 x 600 these days.

Keep in mind that if you want to display a 30x30 tile mosaic on a screen with a resolution of 1024x768, every tile will be about 34 pixels in width.

2.  If you want to print a mosaic on an ink-jet printer:
Even if you have a "very hi-tech super-duper photo" printer that prints 2400x2400 dpi (so they claim in the users manual) you usually do not need an image file with a resolution of more than 150 to 200 dpi. If you want to print an A3-sized mosaic (about 12x17 inches) with a resolution of 200 dpi, you'll need to have an image file of 2400x3400 pixels.

If that mosaic is 30x30 tiles, every tile must be about 80 x 113 pixels.

3.  If you want to make a mosaic suitable for a typography process:
For typography, you'll most likely need a file with a resolution of 250x300 (maybe more) pixels per inch. Consult with your pre-press bureau manager about file resolution and then calculate the tile size you need using the examples from the ink-jet printer.

Note 1
If you want a very high resolution mosaic, be sure that every image you'll want to use as a tile is not less than the size of a tile you want in the output mosaic.

Note 2
When making those high resolution mosaics, it is better to render it first using a small output size and when you are satisfied with the result, render it as big tiff or bmp file. This way you may speed up the overall process significantly.