Don’t feel silly at all!!
For me the first thing they asked was what size was I after (before giving the spiel that they can never guarantee a size) then they exam you to see what size your breast pocket width is to see exactly what they can fit in there.
For each width there is typically about 4 projections of implant that can be offered, low, moderate plus, high and ultra high. You then get to try on the sizers that correspond to this.
For me, I said I wanted to be a C from an A so I only tried on the moderate and high implants for my size which were 255cc and 315cc. If I’d wanted to be bigger, the largest I could have would of been high profile 380cc, as any bigger wouldn’t of fitted in my pocket. So essentially it’s what the consultant measures your body as, and then your preference in projection plus a possible bit of leeway.
Hope all that makes sense?