Rita, You want larger (magnifying everything) AND, assuming portrait orientation is your monitor goal, you want as many vertical lines as you can get so that they equate to horizontal resolution when rotated 90 degrees. A 1080 line monitor may suck when rotated for 6 point fonts and 1200 line (x1920 or better) may be hard to find now days but would help a lot. The good thing about HD tv is that it lowered panel prices - the bad part is that it pushed away vendors catering to higher computer resolution monitors that use to be big and relatively cheap - now they are rarer and more expensive for those of us who deamnd 1200 line displays or better.

Also you can get monitor stands that allow rotation for displays that otherwise don't have it (I use 3M stands for this - though the caveat is that a non rotation aware monitor will need to be set up in the video card settings for proper orientation).
It's not rocket science but it would be easier if you could find someone to show you all this (literally seeing what is good enough) and to answer questions 1 on 1.

Cheapest answer is to be conventional and keep the high horizontal res of a portrait monitor (i.e. 1920 colums) and scroll to see the bottom vs top of a page - scrolling is key-click fast). 20" isn';t adequate for anything unless you have the eyesight of a kid ;-)

Good luck,
B