The workshop has 50 units of space. Each unit can accommodate one working engineer, although each manufacturing project requires workshop space as well.
Workshop = Manufacturing = Money. Never have idle engineers, have them make something (anything) so you can sell it, even if you don't need it. Just be careful of using raw alien materials - Zrbite, for example, is too precious to convert into cash.
The Gauss Cannon is the most profitable project you can mass produce. If you take the cost of materials (Aqua Plastics) into account, the P.W.T. Cannon is about 25% less profitable. See here for details.
Construction Time: 32 days
Construction Cost: $800,000
Maintenance Cost: $35,000/month