Tas II mentioned "limited duration", by which I assume he meant limiting the number of shots/salvos that your little Size 2 missile tender has available. Are they firing all off at once? In other words: Sure a ship this size can do 4-pont missile attacks, but how many times? Drawing deckplans I can tell you that there is not a whole lot of space available for full sized torpedoes in the smaller vessels. How many salvos can it have in reserve then?
This is not something (okay I think this is something, been a whilke since I read those rules) that is taken into account by the rules and maybe that isn't such a bad thing. That way each GM can set this up the way they are most comfortable. Some GM's may want to just let it ride and forget any kind of bookeeping in this way, which is cool depending on the way they envision their setting. Others might only limit the number of salvos for smaller or non-combat type vessels (like the Nova or a courier not intended for heavy-fighting). Some might want to do all the bookeeping and lay out a chart for how many missiles are fired for each damage level (1-point attack uses a single missile, a 2-point attack uses 3, a 3-point attack uses 5 etc.) then tick it off from the total number of missiles they think each vessel has. I'd say that for a vessel that size a 4/4/4/4/0 missile salvo is one shot, if they want two they'd have to split up the damage (one 3-pointer and one 1-pointer for example). You could encourage larger ships to conserve their torpedoes that way as well by making deals at dramatically appropriate moments without too much bookkeeping (Okay you can choose whether or not you do full damage and I mark off one less available shot or you do half damage and we say you have saved enough torpedoes in reserve for another complete volley).
"If you haven't got anything nice to say about anybody, come sit next to me."
- Alice Roosevelt Longworth