Viewing distance is most definitely a personal preference. In home theater with HD viewing and DVD as the primary source, THX standards are used which is about 1.5x the screen WIDTH. So, at 12 feet, you will very often find a 100" wide screen on the wall. This is using industry standards and requires a top notch source.
But, the resolution of the display is 1280x720 which is likely what the Sony will be. Screen door/pixelization disappears usually around 1x screen width. So, that should not be the concern really...
Except, regular TV will look poor. No matter what, regular TV just doesn't hold a candle to DVD or HDTV. So, you have to really ask about how much regular TV viewing you will be doing, and how important that image quality is to you.
Some people watch 50% or more of regular TV... BUT - They have decided that they want the bigger image because they don't care that much about regular TV and want the biggest possible image for their highest quality sources. They simply live with, and accept, the poor image quality while watching regular TV.
Personal decision? You better believe it! But, I would not hesitate for a second to recommend the 50" display, or perhaps something even larger. 42" may be enough for you, but I don't think you will be able to get the full impact of HDTV or DVD with such a small size. Bigger isn't always better, but you have a lot bigger you can go before it becomes a huge issue.
Very much personal preference though.