Correct shell length is check by standing in the bare shell (no liner), sliding your toes until they just touch the front and measuring the amount of space behind the heel. You should have about 1-2 fingers back there, 1 being a performance fit and 2 being a comfort fit.
If the shell size is too large, or you are in the correct shell size but the boots are too wide, you will be cranking the instep buckle pretty hard, which will result in a pressure point around that buckle and a loss of circulation. A correct-fitting boot does not need to be buckled hard at all. When buckled, the boots should feel equally tight in all areas, and not just under the buckles. They should feel relatively tight without even having to do them up. The buckles should mostly do the job of closing the shell around you foot, not tightening the shell.
In terms of order of buckling, there are many versions out there, but the common idea is to have the heel seated properly in the boot. I usually start with the lower ankle buckle, followed by the upper ankle and power strap. Then, I stand up and flex, and then do the instep buckle followed by the toe buckle.