Choosing the right type of monitor can be more complex of a question than just buy the best and biggest you can in your budget. First off, what kind of design work is planned to be done on this computer? If it is critical that all colors must match up perfectly with other computers in the business and all printed media, you need to look at getting a higher quality monitor specifically made for design purposes, mainly using what is called IPS LCD panels. These are pretty expensive, but worth the money.
If that's not the case and you just need a good size monitor to design projects with, then you can go for cheaper TFT LCD panels and even perhaps an LED lit panel. Using a TV for a monitor can be iffy depending upon the TV that you use. Many have strange refresh rates or limited connection options for a computer to work with. If you want to get something smaller than 28-inches, I'd suggest getting an actual monitor. If you want something larger than that, go with a TV.
I recommend anymore if you are getting a monitor that you intend to use regularly (such as daily design work!) then go for something at least 22 inches. That is kind of the best price range right now as buying a smaller monitor is going to cost about the same, but prices start going up pretty quickly when you get larger than 24 inches. Always look for something that supports at least 1920 X 1080 resolution. You may not want the resolution set quite that high in the end, but it's better to have the ability to set it that size for ease of use and larger working area than to not have it and wish you had.
