Please help with slip

I am trying to figure out what size slip I need for my wedding dress so I can purchase the slip online. I bought my dress from Davids Bridal and it is a size 2, there are a couple slips that are size 4 on ebay. Does anyone know if a size 4 slip will work with a size 2 dress? I tried a two tiered slip on when I bought my dress but I can't remember what size it was.

TIA

My slip is one size smaller than my dress but I don't know if that helps...They gave it to me, I put it on, I took it. I'm pretty sure that's because my dress is bigger because I'm busty and it worked out better to go with a corset, etc. Aren't the slips supposed to be the same size?

I'm probably not helping much, sorry