ATM newbie questions

["1000m" <1000m@netzero.net>]

I will be starting my first project as soon as the parts arrive. I am getting a Meade 10" f/4.5 with a 2.6" diagonal. I do not understand how you can use different sizes of secondary mirrors and still get the entire cone of light, being that they are the same distance from the primary. Will this secondary be too big? If I were to use a smaller one would I lose some light?(I dont want to do this because I cant afford it now, but maybe later) and does this mean i am really only getting the light equal to a 7.4" mirror with no obstruction? Any help would be appreciated. Mark