How to measure genome size? - Before sequencing an eukarotic genome, it needs to know the size (Feb/23/2010 )

Hello every one!

We want to make a BAC library of haploid genome of a hydroid. I can collect good DNA from sperm of this species. However, I don't know how to determine the size of the genome in order to prepare enough clones for 10X coverage.
Has anyone experienced with the technique? Please give me your ideas.
Thanks

check the Genome Size database first, and T. Ryan Gregory's work.