So this is a subject which has come up before and the "why" usually gets met with a lot of speculation and I don't knows, so I was wondering if we could put together a thread of what we do know about crossbows in medieval Europe and what is backed up by evidence.

What were the typical dimensions of a medieval crossbow?

What were the typical weights and dimensions of their projectiles?

Is there any evidence that the draw length on crossbows increased or decreased over time?

Are there clear differences between the dimensions of hunting and military crossbows?

How about infantry and cavalry crossbows?

How about steel and composite crossbows?

Anything else you want to add

Not my usual topic of expertise but I was very recently illuminated on the subject by some very interesting videos by Tod Todeschini from Tod's stuff, I'd link them here if I wasn't at work, but some rooting around his channel may help you if you want more information than I can provide.

Nonetheless I'll try to answer your questions as best I can.

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What were the typical dimensions of a medieval crossbow?

In europe most crossbows between 1300-1500 ranged from around 70cm to a meter, but there's a lot of variance to this as much of it is stock length. The limbs I believe were around 20-30 centimeter depending on the type of crossbow, but again, a lot of variance on this as different crossbows were for different purposes.

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What were the typical weights and dimensions of their projectiles?

Crossbow bolts tended to be shorter and stouter (6-8 inches) than arrows as there is a significantly lower power stroke and significantly more force pushing against it than a longbow, meaning a lighter built bolt could shatter from the force if it wasn't solid enough. There was a source that suggested around a 3 oz. bolt for a seige crossbow (~1000lb draw weight) but for smaller crossbows designed for field combat and hunting I'd say around 1.5-2 oz was typical. While bolts typically matched the powerstroke of the crossbow they were used for it is not unheard of for longer bolts to be used, with the tip "hanging" over the end of the crossbow by possibly an inch - inch and a half -. Particularly in later periods where clips that hold the bolt in the channel before it's fired become prevalent, before this adaptation it was probably the done thing to hold the bolt with your thumb if you needed to aim downwards (and risk the bolt falling off the bow).

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Is there any evidence that the draw length on crossbows increased or decreased over time?

I don't have any hard figures on this, but I should imagine circumstantially, that the power stroke on crossbows increased as the understanding and quality of metallurgy increased during the renaissance, as the main reason it was so small (around 6.5 inches) on medieval crossbows is that the steel used for the limbs wasn't of consistent quality, and given the choice between a slightly lower power output and getting a kilogram of broken crossbow limb winging you in the head after it snaps, they made a compromise. Tod estimates that his 1250lb windlass could feasibly be drawn back to 7.5-8 inches but that he would be reluctant to do it (understandable given the risk vs reward).

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Are there clear differences between the dimensions of hunting and military crossbows?

Again, typically, hunting crossbows tended to be more compact and lighter as they were used when riding and carried on foot when searching for quarry, whereas siege and battlefield crossbows were more likely to be used from static positions, and tended to be higher in draw weight too. Hunting crossbows from the late medieval and renaissance tend to be in the 450 - 600lb weight while war crossbows range from around 700-1300lb.

There was also a bit of variance on the method used to span them, with the invention of the cranequin at around (1490-1500), it became very popular in Italy and Germany for both hunting and war crossbows, however in England and France the windlass devices tended to be more popular with the exception of certain affluent members of the nobility who used the cranequin for their hunting crossbows as a novelty and for convenience (As it only requires one hand to use). This is likely because cranequin devices are difficult, expensive and labour intensive to produce.

I'm not sure on the dimensions of steel vs composite crossbows but I can tell you that Tod concluded that Steel limbs are incredibly inefficient for power generation, which necessitated the colossal draw weights of later large crossbows, as they lose a lot of stored energy as heat and can't have a large draw length because of the aforementioned reason. His 450 lb hunting crossbow actually only put out 45 joules of energy compared to his 95 lb English Longbow which generated 54, both of which were trumped by his modern 175lb crossbow with fiberglass limbs and a much lighter quarrel which generated around 124 joules and had a projectile travelling nearly 3 times faster than both. And all three of them put out far less energy than even a 22. long rifle round.

Hope that's answered some of your questions, also anyone feel free to correct me if I've answered anything in error.