On the subject of size, I use an LG C9 55" as a desktop PC monitor. I've measured the distance between the screen and my eyes at 70cm, and it doesn't feel uncomfortable at all. I checked with my optometrist about whether there are any issues with sitting at this distance, and he said it would be fine. Note that I also use an adjustable desk which allows me to lower the screen so that my eyes are dead centre of the screen. Overall, I'm blown away by the immersion offered with a display that fills so much of my field of view. If you're worried about it being too big, don't - if the 55" is comfortable, the 48" will be. I wouldn't go any bigger than 55" though, unless you can get a curved screen. If LG released a curved 65" display, I'd be first in line!
As for image retention, I've racked up 2000 hours or so in the past year (I game a LOT!) and have absolutely zero retention that I can see.
I should point out that I also have the LG CX 77" in my living room, which I use for both video content viewing, and gaming from the couch (I use the Xbox Elite 2 controller, and a wireless mouse/keyboard combo), but am having major issues because I didn't have the right cable. I've got a 15m Ruipro 48Gbps on the way today which should fix that.