Well, there's a few issues that need to be addressed here. We've talked about it, and it makes _no_ sense to limit maximum top speed, from a physical standpoint. There just isn't. However, with that having been said, it also is not practical to dogfight at 10,000,000m/s. So, here's the deal. Most ships will limit the maximum speed they're going at on a sort of slide scale. This way, the pilot can just kick the engine on full thrust, and get away if he needs too, but under normal circumstances his ship will maintain the requested speed he wants. For most dogfighting it will be around 80 to 200 m/s. That seems to 'fit' best.
So, here's the dilemma: Without a hard set maximum top speed, your going to have a difficult time deciding on the right size for things. Secondly, I was thinking 1 CS unit is 1 meter. (IIRC we can have floating point lengths for things, so this isn't an issue for the modelers.) So, working with 1 meter and a max speed of several thousand meters per second (or even no max speed at all) we've got an issue. Say the target speed is about 200m/s. Fast ships will be in the 250-320 range, and slow ships will be in the 80-120 range. We'll allow people to go faster, but they won't be able to dogfight very well.
I hope this gives you something better to work with.
--Chris