A variety of sizes. and styles.
It may seem expensive to carry a huge range of sizes and styles, but consider the profit you are losing by not catering to those who don’t fit the SML/MED/LRG sizes. Most likely your fans come in all shapes and sizes and providing options for them is vital. Not only that, but imagine their delight when they finally find a band that carries their size. I would personally suggest carrying XS-2XL. It is inclusive of your thinner than average and thicker than average fans without breaking the bank.*Keep track of what sizes sell the best and what styles.
Shapes. Necklines. Hem lengths.
This should seem obvious but don’t force your female fans to wear men’s clothing. If they want to great, but give them the option. When you have boobs, the necklines on men’s shirts are extremely irritating and never sit properly. Shirts can look dumpy. The chest area is stretched, tight and uncomfortable and your arms and torso are swimming in fabric. It’s unappealing and either A, the shirt won’t get worn after the concert. Or B, while wearing it at the concert other girls will see that it looks like crap and won’t want to buy it. Another thing to watch out for is the length of the shirt. You don’t want a shirt that resembles a dress or a crop top unless that’s what you are going for.
Make it wearable.
If there is one thing I hate it’s those band shirts that have people’s faces plastered on the front with a band name and perhaps tour dates. The shirt usually doesn’t fit properly, is … shall we say less than sexy, and is so obviously a tour shirt. No one will wear this when they go out. Maybe high schoolers to show off that they went to the concert but no adult will wear this on a regular basis. Make a nice shirt. That’s it. A nice shirt. Having your band name on the shirt is great. But is it a nice shirt? That is the question. Also take note of the age of your buyers.
If you play mostly all ages shows and kids buy your shit the most, take a moment to consider the appropriateness of your design. After googleing ugly band merch I came across this crude picture of shirt that features a rhino humping the shoulder of another rhino as filth and shit comes out of the (receiving) rhino’s ass. A, it makes no sense at all. Zero. B, It is ugly and disgusting. C, Kids will probably get in shit (pun intended) for wearing it to school and parents might burn it lol. Consider the image you are sending out and if it is wearer friendly. If people can’t wear it anywhere no one will see it.