[rock jock]

2.5" or 4"? What are the advantages/disadvantages to each? I am inclined to just get the 4" model since it will offer more shade and will not obstruct anything out front.

[WhiteKnight]

A sunshade reduces the light transmission of your scope. Hence for prarie dog shooting during the sun's brightest hours, in the middle of the day, a sunshade is perfect. However to a deer hunter in the early morning or late evening, the sunshade cuts down light and impairs the brightness of those late-day shots.

[B27]

You may be surprised at how much perceived length the 2.5 inch one adds.
This 3.5-10x50 has a 2.5 inch shade on it-

http://www.fototime.com/6451D43D0E77D12/standard.jpg

[rock jock]

Good point, whitenight. This is for a scope on a Varminter and I may just wait until I need one.