I myself wouldnt buy a bike without testing it but if it helps:
My wife rides a 51 mens Bianchi very comfortably for 6 years, I look into a WSD bike for her. She's 5'4 with a short torso. I went to a guy that I trust, matter of fact the only guy I trust of the several shops around. I tell him she's a 51 and wanted to try a 51 WSD. He says no, she's a 49 just by looking at her and knowing the WSD set up.
At anothr shop, Dopey said the 51 was the same in the 51 WSD so I thought my trusted friend had to be wrong! I thought he was going crazy since Dopey at the other shop had said 51, same as a mans bike. SO my trusted buddy says that he hasn't any in stock but to try another store with a large inventory. I do and my wife testrides some WSD bikes. She ends up with a Trek 5.2 full carbon Pilot.
Guess what size? 49 just as my trusted buddy had suggested
I told my budy about the deal and asked if he could match it cause I would rather buy from him. He said "buy the bike, it's too good of a deal to pass up".
We took the bike home and mathce dit up withthe mans 51, the handlebars are the same height as the mans 51, if not a tad bit higher. So eventhough it is sized smaller, it isn't in actual size. bu the WSD is much more comfortable for her.
So if you ride a 50 in a mans frame like my wife, but the shop says you need a 48 in the WSD, I believe it!
But like I said, me myself would not buy a bike that I or my wife could not test ride. But that's true for anything that I buy!
Somethings ar worth the 100 mile drive. Heck we drive a hundred miles to other shops in other areas for funzies sometimes. Plenty of times just to look without buying so much as a tube.
BTW,wife is 5'4 and rides a 49 WSD Trek Pilot.
Wifes' 49 WSD and her 51 Bianchi (traditional mens frame)..I know you probably can't tell by the pics but trust me, the handlbars are the same height. Checkout the hb height compared to the couch, about the same.