To allow for the cloaked Romulan mines and birds-of-prey seen in "Minefield" [ENT], I've worked up stats for three early-model cloaking systems. Availability dates assume the systems are of Romulan origin (so no need to adjust them by 20 years).

Class A Cloak:
SPACE: 1 + half Size
RATING: 10
MAXIMUM SIZE: 2
AVAILABILITY: 2113
NOTES: The Class A cloak is a prototype that the Romulans never found practically useful. It would take multiple generators to cloak any vessel larger than a shuttlecraft and the cloaking field was very "leaky."

Class B Cloak:
SPACE: 2 + half Size
RATING: 12
MAXIMUM SIZE: 3
AVAILABILITY: 2127
NOTES: The Class B cloak is the model used in the cloaked mines. Really the first practical cloaking device, but still unable to cloak a capital ship without multiple cloaking generators installed. The cloaking field still leaks some EM and subspace frequencies and wavelengths, but the system offers practical protection against standard scans.

Class 0 Cloak:
SPACE: Size
RATING: 14
MAXIMUM SIZE: 5
AVAILABILITY: 2140
NOTES: The cloaking system the Romulan Star Navy was waiting for, the Class 0 offers an adequately secure cloaking field and can be fielded on smaller capital ships with only one generator. This is the model installed on the early birds-of-prey seen in "Minefield" [ENT].