Pick a Mesh Count: 
110: Special Effects Inks, Plastisol, Glue, White Ink
137-160: Most versatile mesh count, holds medium to high detail with great ink coverage. 

NOTE: As you go up in mesh count water based inks dry out more quickly with smaller holes. Add retarder to water based inks / reducer to plastisol

195: fine detail, haftones up to 39 lpi
230-280: Paper! intricate designs, 2-3 pixels, halftones up to 46-55 lpi. 
305: Four color process printing, CMYK, very high detail, halftones up to 55-65 lpi 

More info from linked site.

Pick the right Screen size for your Image size: Add 8" to the length and width of your image.

Screen Size Image Size Price
20"x24"
12" x 16"
$45
23"x31"
15" x 23"
$60
25"x36"
17" x 28"
$72

You will need a separate screen for each color