First, it is "Marcye". Second, you probably talked to Francesca(don't know if this is correct spelling).
With the height of your tank, you could put enough rock formations to have a nice bristletooth tang, a zebrasoma, some acanthurus species. There are easily some good choice for tangs that will fit fine in a 90G for at least a couple/few years. Yes, tangs do grow large, many over a foot in length, but if you buy as a juvinile, you should be fine with your size for a couple years before deciding either to give it away or compensate for its size
I know people are going to come on here and say that I am bad for doing this, but I have two bristletooths in a 55G, one about 5", one about 3" and they are doing absolutely perfect. They are fat and healthy. They are swimming fine and always picking at the rocks. They are not showing any signs of stress. Yes, in the next few months I will be giving them a much larger home(180G) but they have been fine for over a year and looking healthy. Truthfully, please don't come here and tell me I'm worng for doing so. Everyone has their own opinion on how a reef should be set up. Once my fish show the first sign that the tank may be too small, then I will decide what needs to be done.
All, that being said, I do not recommend a tang for a 55G(even though it works fine for me), just like I will not tell people PC lighting is okay for a BTA(although this has been my best BTA tank of all of my three clones; it seems that the BTA likes it better than the MH lighting). Although it can work, I don't like being held responsible for others misfortunes, hence the reason I do not recommend...I just tell what has worked for me.
I would think a 90 for any juvi tang should be fine, for most small adults should be fine again. Just don't get a tang that grows exceptionally fast or exceptionally big and a 48" length with a 18" width and 24" height should be perfectly fine. I'm sure if you talk to Marcye and release her of any responsibility, explaining that you won't hold it against her if something happens, then she may be okay with selling you a tang for your 90G. Put some time and research into them before you get one to make sure you feel comfortable with the idea of keeping it in that tank.