I am in the process of starting a solo VBAM/Starmada AE campaign (lot of rules to sort through, though) and have considered using a system/convention like this:
CT- 1
FG- 2
DD- 3-5
CL- 6-8
CM- 9-11
CA- 12-14
BC- 15-17
BCH- 18-20
BB 21-25
DN 26-30
DNH 31-35
JG 36-40
LN 41-45
BS 46-50
WS 51+
Carriers: Just a classification, a size class 12 ship is still a CA, just with carrier functionality
CVE 3-11
CVS 12-20
CVL 21-30
CVA 31-40
CVF 41-50
*A ship bigger than 50 is still just called a World Ship, even with Carrier functionality. Also note, the convention of just adding a "V" could be used: DDV, CLV, BBV, DNV, etc.
Bases
DEFSat 1-2
CP 3-5
SAMS 6-8
Operations Base 9-11
Mobile Base 12-14
Base Station 15-20
Battle Station 21-30
Sector Base 31-40
Star Base 41-50
Star Fortress 51+
Access to each range for ships and bases would be dependent upon starting tech level. For instance: Interstellar-1 (just achieved FTL capability and can expand from home system) would start out with ship size classes 1,2,3 and 6. They would have the option to further research the DD and CL range (4 and/or 7) as well as the Medium Cruiser range (9)- the ranges (or first size class) must be researched in order but different factions could have, say, a larger CA class capability than others who haven't researched further into it. So their research orders for the period (Per VBAM/Starmada X) might look something like this:
1- DD size class 4
2- CL size class 7
3- CM size class 9 (the largest a faction currently possessing CL's could research)
4- Anti-fighter batteries
5- Scout function (2)
Each range for ships (and bases) has the same maintenance costs. I'm also looking at the possibility of being able to refit ships/bases in the same range. So that a faction that successfully researches, say, BC size class 17 could refit its remaining size class 15 and 16 BC's to a new Size class 17 BC it designs. If it wanted to.