Each Attack Craft squadron should be a 20mm square base.
Torpedoes are a little trickier. In the classic rules, you had different templates for different numbers of torpedoes, IIRC in groupings of 1-6 (and you would simply combine them for larger values). In the FAQ 2010, this was changed so that all torpedoes use the 3-strength torpedo template, with dice to indicate actual strength. Some disagreed with this change, however.
The 3-torpedo template is approximately 24mm x 20mm if memory serves. So an individual torpedo is 8mm wide and 20mm long.