@Japa: Since DF produces clothing for off the shelf pickup, changing the model to custom production would be a fairly significant change, although that would be more historically accurate. That would basically change the tailor job from being an industrial production job to being an on demand job (like butchering). Urist would order a clothing item and the tailor would produce it, after which Urist would pick it up (and probably order a new item). The player would then have to guess if the cloth production was sufficient and if there are enough tailors (unless the jobs list would display the backlog), you couldn't produce clothing for export (although that's not really needed currently, as the worn stuff is more than sufficient), and you couldn't equip your dorfs with the highest quality clothing unless you could set the tailor to retry (i.e. produce new items) until a player set quality threshold was reached (similar to the book keeper's accuracy), but you'd get problems with paired items of independent quality, which would probably have to be de-paired, at least at the production level. I believe historically, although clothing was "custom" made, it typically was of a general enough fit to be inherited by someone of generally the same size (frequently regardless of gender) until worn out (only the wealthiest had money to waste on tailoring for themselves exclusively). In a multi species fortress I think the S/M/L sizes should be sufficient (unless you need XL for ogres/giants).
Another issue is with armor, where you may very well WANT to pre produce to have it available when you get your recruits (and to train those producing it). It'd get messy to have one logic for leather gloves and another for leather armor. Also, allowing Urist to order items would also mean you don't have control over the material used (what kind of cloth, or leather), although I guess a selection to allow/forbid leather for clothing might work. Such a system might allow Urist to ask for a specific material/color if available, but in that case the item received would have to be a "best fit" based on what's available when the item is produced (if Urist is keen on wearing "something special now and again" he might be more likely to try to order additional stuff to better fit his taste when the materials are available, possibly with a "don't bother if you can't match my request" for these extra luxury items to avoid a huge pile of unnecessary luxury items).
Would the above be worth the effort to implement? Probably not.