SpanxShapewear for Women, Tummy Control Underwear Everyday Essential Shaping Women's Knickers Brief

I need fat belly support lol. I’m slim but with a belly and have a gorgeous tight dress for my anniversary. Would these smooth/hold a tummy in dress

Can someone give guidance on sizing for large and xl please?

Can these underwear be returned if they dont fit

I'm between a size 12 - 14 will the size 40 fit ok?