The ships of the Royal Navy were classified (rated) into six categories according to the number of cannon or guns that they carried; which, generally, by this method, sorted the vessels by size. It was Lord Anson, during the years 1751-6, who first set up the rating system by which English warships were described for purposes of battle. The largest of the war ships, first rate, carried 100 or more guns; second rate, 84; third rate, 70; fourth rate, 50; fifth rate, 32; and sixth rate 32 guns or less. (See Oxford Companion to Ships & the Sea.)

The HRS Rose was a frigate, but only had 24 guns, so it was a sixth rate ship.

Sloops, schooners, and brigantines were merchant ships, distinguished more by rigging and design than by size. The Gaspee was much much smaller than the Rose, was a schooner, and only had eight guns.  It was also a sixth rate ship, but so small that it was not designated as "HRS" (His Royal Highness's).  Sloops and schooners were built to haul freight, and were common vessels in the coastal trade.   Sloops had one mast; schooners, two.

You may want a description of the class of ships known as a "brigantine".  It is larger than the ordinary sloop and schooners used in the Rhode Island trading ventures.  Joseph  Bucklin 4th was master of the brigantine Providence when it was seized for running smuggled goods.  The brigantine Providence which was owned before the Revolution by Stephen Hopkins , and by Joseph Bucklin 4th, was owned after the Revolution by Joseph Brown 

The ship's longboat was the the longest boat carried on a sailing ship. Hence the size of a longboat depended on the size of the ship that carried it, and the use intended for the boat. We have described whaleboats and longboats for you, because they were used in the attack on the Gaspee.

If you want to get a flavor of what the outnumbered sailors on the English ship felt like when attacked by a hundred or so men in ten different longboats, loaded with 108 or more men, you should understand the small size of the schooner Gaspee.