ARRL Antenna Book has all the answers.

Shortening the antenna will reduce the effective radiated power and will reduce the amount of gathering ability of the antenna.
Every time you reduce the size by 25% - you throw away - probably 50% of it's ability.

Antenna's are always a compromise when you get into the lower ranges.
It is easier to BUY a Mosley tribander - about $1000.00, then to try to make your own.

As the elements are shortened or lengthened, all of the variables has to be recalculated because the spacing will also change and the boom length.

http://fermi.la.asu.edu/w9cf/yagipub/index.html

http://www.ac6v.com/antsoftware.htm