Guide to reactors: anything between 1 and 5x5 is completely worthless, 5x5 might also be shit, but I don't have any ships of that size.
(pictured, as of Dev v0.200.119)
7, 15, 27, and 93, reactors
2, 8, 22, and 88 stabilizers
2, 7, 10, and 18 required space