I've just posted an article over at miniwargaming.com about magnetizing a terminator base. Login is required, but its free.
The magnets I used (and links to where I buy mine) are:
1/16" x 1/32" Magnets (2 per terminator)
1/8" x 1/16" Magnets (2 per terminator)
1/4" x 1/16" Magnets (Optional, 1 per terminator)