Originally Posted by Kacee

If you're talking about this one the dimensions (13.75 x 16.25 x 8.5) are larger than UA's "personal item" allowance (9 x 10 x 17).

If OP's question relates to basic economy restrictions, they don't actually stop you from using the overhead, they just won't let you carry-on a bag that exceeds the personal item dimenstions. I don't know how much play there is in the sizer, but 3.75 inches is a pretty significant overage.