The standard size for full front printing on t-shirts is 12″x12″ with an offset of 3.5″ off the neck line. If you also have youth sizes in the run that size would be way too big so you either have to run two sizes (and charge for two setups) or explain upfront why the adult prints look kind of small. I’ve done contract decorating for the last 12 years and by far left chest is the more common small front placement, 5″down and 4″ over from the center of the neckline. In adult male mediums or smaller center the design between the shoulder seam and the center line of the shirt.On the big ones even 6 7 or 8X don’t move the design out more than 5″, it will get lost in all that fabric. Ladies prefer the design up and inch or so, for discretion. Right chest placement is not uncommon and is done a lot for pocket Tees.
Left sleeve is also more common than right with the designs baseline offset 1/2″ off the sleeves seam. Having said that these are just generalizations in the shops I’ve worked in. And clients love to say whats what on any occasion (typically), and will assume that what they say is common practice.