This is a tricky question. Among reenactors and other enthusiasts there is a kind of modern folk wisdom that 'real' armour requires true tailor fitting and extensive customization - the 'bespoke' approach of extensive measurement, custom patterning and multiple fittings. And we can document this kind of close personalized for some plate armour in the late Middle Ages and Early Modern period - that of the upper reaches of the Military aristocracy - royalty and titled nobility. We have records of men at arms (knights and those who fought as knights) getting fitted for armour (as John Paston did with Martin Rondele, armourer to the Bastard of Burgundy) and other cases of kings or nobles sending their doublets and other closely fitted pieces of clothing to their armourers for reference if they could not be fit in person. In the case of Kings, they would often have court favorite armourers who might have measurements stored, or who might even travel with them to measure and fit, as Kolman Helmschsmidt travelled with Charles V.

However, this armour - truly custom, tailored armour - is only the very highest end of the armour industry, just as bespoke clothing today is the most exclusive, expensive and rarest form of menswear. Even among those who could afford plate armour and warhorses this kind of truly custom armour would be a luxury. In a time when most fully armoured soldiers didn't have noble titles but belonged to the larger and less well defined class of the 'gentry', custom armour was mostly the purview of knights, barons, earls, dukes and counts, or members of royal families and their favorites. Your average armoured horseman would get something that was more 'off the rack', though it could still be of decent quality and would still be a full harness. We see this vast midrange of armour all over the place in written records - bought by the single harness inthe Howard accounts, or by the hundred or more by Edward IV of England. We know that this armour wasn't custom made because it's sold semi-generically as 'a harness' and no provision is made beforehand for conveying measurements. This armour was often imported from one of the great armouring centers like Milan (in the case of England, France, Aragon) or possibly made in Nuremberg (in the German lands or points east) or in a local armouring center or another center of manufacture like the low countries. This was the 'mid range' armour, and it probably equipped the vast majority of men at arms. I don't have numbers for it, but given the prices (at least 2 or 3 pounds for a full harness, several month's wages, even for a man at arms) of these armours must have made up one of the most profitable segments of the armouring industry - good enough to make serious money, but still capable of mass production if you divided the construction of different pieces between different armourers. This armour might be bought new, or it might be used - we know from the diary of the 14th century merchant Francesco Datini that merchants would literally follow armies and seek to buy up looted armour for resale.

Now, there's still the question of fitting, even in armour that wasn't originally fitted to the wearer. Armour doesn't work very well if it doesn't fit, and some parts of it, like the greaves, need a fairly precise fit or they hardly work at all (unlike other parts of armour, the greave and demigreave above it cover the whole shin and calf, and so have to be the right length, with only a little margin of error). However, just because armour was bought 'off the rack' doesn't mean it wasn't adjusted. We know that among armourers there was a lower-paid class of craftsman often called furbishers who repaired armour (and weapons), and it's possible that these may have adjusted armour to fit its new owners.