You would probably be better off learning the limitations of the designer by experimentation, because the rules have a lot of conditionals and exceptions.
An inexhaustive listing of design limitations by class:
DDs:
- Must carry at least one gun, at least one torpedo tube, and have a design speed of at least 21 knots.
- Cannot be armored or carry guns heavier than 6".
CLs:
- All CLs must be at least 2100 tons but not more than 8000 tons nominal displacement and have a design speed of at least 17 knots.
- A CL's main battery armament is limited by its armor scheme:
-- CLs using the 'protected cruiser' armor scheme (hereafter 'protected cruisers') may carry up to four 7" or 8" guns or one gun of up to 13" caliber, and may have at most two turrets on the centerline.
-- CLs using the 'sloped deck' or 'flat deck' armor scheme (hereafter 'light armored cruisers') cannot carry guns heavier than 6" and may carry as many guns on the centerline as technology allows.
-- Prior to the development of double gun mounts for CLs, the two centerline turrets allowed to a protected cruiser can each house two guns; all other CL main battery guns must be individually mounted whether on a protected cruiser or a light armored cruiser.
-- After the development of double gun mounts for CLs, all CL centerline turrets (including those on protected cruisers) may house up to three 6" guns and all CL wing turrets may house up to two 6" guns.
CAs:
- Must be at least 4100 tons nominal displacement and at least 18 knots design speed.
- May carry heavy (11" or larger) guns if:
-- Their nominal displacement does not exceed 10,000 tons and their belt armor is no thicker than 6.5".
-- Their nominal displacement exceeds 10,000 tons, their belt armor does not exceed 6", and they carry either one heavy gun or their design speed does not exceed 22 knots (up to two turrets housing a total of four or more heavy guns) or 23 knots (up to two turrets containing a total of three heavy guns).
- Must have at least:
-- 2.5" belt armor if larger than 8000 tons nominal displacement or carrying main battery guns heavier than 6".
-- 3.5" belt armor if no larger than 8000 tons nominal displacement and carrying 6" main battery guns.
Bs:
- Must be at least 5100 tons nominal displacement with at least 6.5" belt armor carrying a main battery armament of at least one 7" or heavier gun and a secondary battery armament of at least four guns.
- Cannot have a design speed in excess of 22 knots
- Cannot normally carry a main battery armament of more than seven heavy guns or in more than two turrets.
BCs and BBs:
- Are distinguished primarily by belt armor thickness and design speed:
-- Prior to 1916, capital ships with a 'traditional' main battery configuration (3+ turrets or 8+ heavy guns) are BBs if the design speed does not exceed 23 knots and are BCs otherwise.
-- From 1916 onwards, capital ships with a traditional main battery configuration are BBs if the design speed does not exceed 30 knots and the belt armor exceeds 12" or if the design speed does not exceed 23 knots, or are BCs if the design speed is 24 to 30 knots and the belt armor is not thicker than 12" or the design speed exceeds 30 knots.
-- Capital ships with fewer than eight guns in up to two turrets can qualify as BCs if they are too fast or too lightly armored to qualify as Bs and are too heavily armed or too heavily armored to be classified as CAs.
- Not sure exactly what the lower limits are, but the lightest BB I was able to design in a quick test was 8100 tons with a 4" belt while the lightest BC I was able to design was 10,100 tons with a 4" belt.
Other:
- AMCs may carry min(80, 2 * ceiling([nominal displacement] / 200)) mines. DDs and CLs will gain the ability to carry mines later in the game, with the same limitation on number carried.
- AMCs must be at least 1800 tons and cannot exceed 21 knots design speed or carry guns heavier than 6".
- Secondary and tertiary battery armament is limited by gun caliber and nominal design displacement.
- A design may use up to 16 main battery turrets with 7" or lighter guns, up to 12 main battery turrets with 8" to 10" guns, or up to 10 main battery turrets with 11" or heavier guns.
- A design may not have more than 7 centerline turrets.
- Main battery wing turrets may only house one or two guns each.
Note: there may be exceptions to the above rules, and I do not claim that this is the complete set of design rules. Also, this is for v1.34b1; if you are playing on another game version, there may be some differences between the rules as they apply in your game and the rules as stated above.
I believe that in v1.34b1 the maximum size of a minesweeper is 1700 tons.
31 knots in v1.34b1.
It doesn't matter if you have more than 2 Ship Design technologies; you're allowed to design CAs with heavy guns of any caliber as long as the ship carries only one main battery gun (any design speed), three or fewer main battery guns in one or two turrets (23 knots maximum), or no more than two turrets (22 knots maximum; there might also be a barrel count limit). If the ship is larger than 10,000 tons nominal displacement, then its belt armor can be no thicker than 6", while if it is no more than 10,000 tons nominal displacement it may have up to 6.5" belt armor. To the best of my knowledge, maximum gun caliber is not limited.
