A 17" screen is a fairly standard specification, and makes for a reasonably priced, useable PC Monitor. Be aware that PC Monitor display size measurements are made from the top-left to the bottom-right corner of the visible display area (the actual screen.)
You may find benefit in a larger Monitor if you plan on using high-resolution graphics applications, for example photo/video-editing packages, CAD/CAM or games.
Your monitor could be either a CRT (standard) type or newer LCD (Flat Panel) style depending on your budget and usage requirements.
Typical Monitor display sizes: 15", 17", 19", 21", 22"
Reputable PC Monitor Brands: Iiyama, Taxan, LG, NEC, Neovo, Sony, Viewsonic
Check the connector type on your motherboard. In the case of the Keyboard this will be AT, PS2 or USB. The Mouse will be either Serial, PS/2 or USB.
Mouse and Keyboard style are personal choice, but be aware that if you will be using your PC for extended periods of time, ergonomic Keyboard and Mouse designs will help prevent conditions such as RSI (Repetitive Stress Injury.)
Reputable Keyboard/Mouse Brands: Logitech, Microsoft, Creative