Upgrading SSD on mid 2014 MB: which SSD? I have a pretty simple issue. 

The SSD of my mi-2014 Macbook pro (retina - MacBookPro11,1) seems to be too short (256GB).

So I'm considering buying a 1Tb SSD for my Macbook, but I read, here and there, that not all SSD are compatible (because of its format, "blade" SSD or something).

So, do you know which 1Tb SSD I could upgrade my Macbook Pro with ?
Without having heard of this sort of incompatibility before, I'd suggest you check out Other World Computing, who have a great reputation for Mac aftermarket parts - including a great line of SSDs. I've been happy with one of their SSDs in an old MacBook Pro myself. Great product backed with a thorough understanding of Apple's product line and fantastic customer service. 

And to be clear, I'm not affiliated in any way other than being a happy customer. 

Update: Thanks to @enguerranws for doing the extra legwork; OWC indeed does have a solution available for the 11,1 in 240GB, 480GB and 1TB as of at least Dec 2016.