How “tailored” (don’t know another word for this) was a knights personal armor to his physique? This is in regards to Garlan wearing Renly’s armor in the Battle of the Blackwater, as well as the opposing knight’s kit won (for ransom) by the victor of a joust. TY Maester Steven.

Depends on the era and the type of armor in question, but when you get to plate armor, it was usually “bespoke” as opposed to “off the rack,” because the whole weight-distribution thing requires that the pieces conform to the dimensions of the individual body. 

However, that doesn’t mean that no one else can wear it – just that the utility of the armor is going to depend on how close you are to the original intended wearer; if you both wear the same “size,” no problem. So as long as Renly and Garlan were roughly the same height/weight/proportions, it would work fine. (Indeed, it’s possible that’s the reason Loras wasn’t wearing the armor – it wouldn’t have fit him right…)

As for the opposing knight’s armor, as the Knight of Snails points out, the armor itself isn’t about to be worn by the victor or even likely to be sold. Armor was so expensive and so important to the livelihood of the knight that it was understood to be a possession you’d redeem it at all costs, and thus it was acceptible collateral for a debt.