DaddyX3 is 100% correct.
20Gig is to small to be setting up different partitions.
Me, I would do exactly the same thing DaddyX3 suggested,
Select guided use whole disk... for single instillation
Select guided install using largest continuous free space... for duel boot
the guided method will let you select the size of your /Swap.
A few months down the road, when you install your next release you will have a better feel for what you need.
(and maybe a bigger hard drive)