I'm curious to know whether or not generic S-M-L clothing was available to them, or whether everything they wore was made to order -- even for the least wealthy. The answer is likely something in between, so I'm perfectly happy to hear about that as well!
Short answer: neither. 'Bespoke' clothing for civilians was not introduced to Europe before the 14th Century; before that time, the fashion (and technology) leaned toward loose, flowing robes, dresses, tunics, etc. 'Off-the-rack' clothing, which is made to standardized patterns and sizes and mass-produced in factories, was not introduced until the early 19th Century. Specifically, ready-to-wear military uniforms were introduced in the United States during the War of 1812, with civilian fashion options for men being introduced shortly thereafter. Women's clothing in the United States remained bespoke until the 1920s. In 1937, the US Department of Agriculture standardized clothing sizes. 'Off-the-rack' clothing technology would spread from the United States to other Western nations in the decade that followed.
TL;DR: Ancient Roman citizens would not have been able to buy things 'off-the-rack'. In fact, their descendants in the 1950s would probably have been the first to live in a world where men and women alike could enjoy such convenience. Ancient Romans wouldn't even be able to buy things 'bespoke', because not even that had been perfected yet. An ordinary citizen, rich or poor, would have worn loose, flowing clothing.
Sources
National Institute of Standards and Technology
Hollander, Anne (1992). "The Modernization of Fashion". Design Quarterly 154: 27–33.
The nature of how Roman and Ancient Greek clothing was designed was based on what loom the cloth was made on. It was very labour intensive to weave fabric so the styles tended to be ones that could be made on the loom without any cut off. This is why you generally had tunics that attached at the shoulders and were belted. This changed overtime with varying styles but generally kept the notion in mind that you didn't want to waste an inch of cloth. Being belted and loose also meant that it was fairly easy to fit to numerous body types. Most upper class women (supposedly Livia wife of Augustus included) liked to be seen as weaving their own cloth. From this we can assume some level of home production. If you are interested in Roman fashion read Roman Clothing and Fashion by Alexandra Croom. I came across this whilst doing my dissertation on the construction of Roman Beauty- interesting topic!
Furthermore, did they even have specific stores for things, or just like one large generic store?