Your underband should have a snug fit and lie parallel to the ground. You should be able fit two fingers comfortably underneath the band.
The broader the sideband, the better the support. In case of a plus size bra, the sideband should be broad enough to provide utmost support & avoid spillage.
Make sure that the centre of your bra between the cups sets lies flat against the chest. The higher the gore, the better fit it offers.
You know you’ve found the right bra size when you don’t feel like you are wearing a bra at all. Wires and seams shouldn’t dig into your breast or underarms. There should not be any top and side spillage from the cups. The cups should properly fit your breasts without any gaping lines or quadra-boobs.
Not all bras are underwired, but the ones that do are usually made of coated metal and situated at the bottom of your bra’s cups to offer gentle lift to the breasts. These wires are enclosed in fabric to keep you poke-free all day long.
The bra straps shouldn’t glide off your shoulder or dig into your skin. They shouldn’t have any hard clasps or plastic bits either.