In the time period I study, sailors began to be issued working clothes (referred to as "slops") by the ship's purser starting in the late 17th century. Via The Command of the Ocean by N.A.M. Rodger:

These slops were supplied by contractors, issued by the purser and charged against men’s wages. In the 1690s they included blue waistcoats and kersey jackets, white ‘petticoat breeches’ (a sort of canvas divided skirt) with red stripes, red caps and white neckcloths. Richard Hamage’s 1706 contract specified grey kersey jackets lined with red cotton, fifteen brass buttons and ‘the buttonholes stitched with gold-coloured thread’; red kersey breeches, red flannel waistcoats, red or striped shag breeches, blue and white check linen shirts, linen drawers, leather caps, grey woollen stockings, gloves and mittens, shoes and brass buckles.

The slops were bought from contractors and issued to men in bales that would contain a variety of sizes of uniform -- the contract above for 1706 had three sizes, while in 1711 the number of sizes increased to four. Each bale of 50 suits had eight of the smallest size, eight of the largest, and seventeen each of the two middle sizes.

Contracting for slops was not a great business for the contractors -- the Navy drew up contracts such that the contractor would be responsible for damage until they were issued, and most of the shipping and storage was out of the contractor's control. In 1709 Harnage was owed £44,553 9s 1d for providing three years of slops, all in arrears. One can imagine that under those conditions, contractors had little incentive to produce well-made clothes or consistently-sized clothes. New contracts for slops were issued periodically, until the Navy Board took over issuing them at their own risk in 1758.

Sailors in any case would generally alter clothing that was provided them (or make their own from scratch -- some pursers just served out materials, especially duck for trousers and sennit for hats, in hot climates). It was considered poor form to wear unaltered slops, and many sailors would decorate their shore-going clothing by sewing ribbons into the seams. The "fit" of the uniforms would vary a bit depending on what the style of the individual ship was. You can see lots of examples in this older thread: https://www.reddit.com/r/AskHistorians/comments/2pdr6j/how_did_the_sailor_suit_develop/