Boats指的船可以最大有多大?

BOAT (艇, 船) 有兩個定義
(1) a small vehicle that people use for travelling on water. Boats are usually smaller than ships, and are moved by means of sails, oars, and motor.
Example: sail-boat, motor-boat, row-boat, speed-boat, tug-boat, pleasure boat, lifeboat, kayak, canoe, yacht, catamaran, barge, dinghy, hydrofoil, hovercraft, punt, cutter, dory, steamer, raft, trawler, schooner

(2) any vessel that travels across the water, including a ship that carries passengers (船的統稱)

SHIP (船)
a very large boat that carries passengers and/or goods, especially over a long distance.
Example: liner, ferry, cargo ship, passenger ship, freighter, tanker, supertanker, icebreaker

In the Navy, a boat is defined as a “vessel that can be hauled aboard a ship”.

In common usage, a ship is somewhat larger, though submarine is always referred as a boat. e.g. U-boat.
However, larger passenger ships which travel short distances are sometimes called boats.

She was getting off at Hamburg to take the boat to Stockholm.

Boat and ship are not interchangeable in some phrases and idioms.
in the same boat, miss the boat, rock the boat, ship of fools, jump ship, run a tight ship

題外:
sampan (中國舢板), junk (中國平底帆船)
gondola (意大利威尼斯兩頭尖的平底船)

ship is bigger than boat

你這樣問是因為你的詞彙仍然只是在統稱的程度

我會這樣回答
大過yacht/中式的junk的便算是ship
細過yacht/中式的junk的便算是boat
但sail boat卻可以很大隻(例如比賽用的sail boat可以載上十幾人)要比最細的yacht還大
專業些的答案可能按排水量，但同樣只能是統稱。
但再小一級的艇仔則不可叫做boat：中式的叫sampan、西式叫dinghy。