Ya I would go with 1/4 inch. 1/2 inch seems way too big for mice and I would be concerned with escapes. I know Home Depot, Menards, and Lowes all carry 1/4 inch hardware cloth.
Mice WILL escape 1/2" hardware cloth. Check your local Co-op for 1/4". They had it cheaper than anywhere I looked and you more options for length and width than HD or Lowes. Good luck.