I have a 15" XGA screened Laptop and find that difficult enough compared to the 17" widescreens i have seen and borrowed recently......
I dont think i could cope with a 12 or 13" but then i suppose it depends how long you intend sitting infront of it and how good your eye sight (how old you are!) is...
I find big laptops cumbersome to lug around and their batteries don't last. I have a 24 inch LCD that I use in vertical mode that I use as a second monitor when programming and siting at my desk.