What is the difference between all the installation size tabs?
It is automatically set at 18 gigs, but it is possible for me to put 30?
What do I choose during installation?
This is the size that the installation of Ubuntu will be allowed to take up; I would recommend increasing it if you can spare the space. You will need at least 5GB or so.
Short Answer: In essence what the installation size refers to is the available space (HDD) you'll have access to when you log into Ubuntu. You can resize the space allocated via the instructions given at https://help.ubuntu.com/community/ResizeandDuplicateWubiDisk. The minimum you should set the installation size to is probably 5GB as you'll easily use up this much space with just random downloads and programs you install.
Alternate Route: You could install Ubuntu on a separate partition. To do this you must place a live CD or a live USB into its respective place with respect to your computer such that it can be mounted onto it. When you do this you must reboot and then from there proceed to install Ubuntu. In the case of Dell Vostro 3500 (my pc) at least, as I'm unaware of other PCs, you have to press F12 before it'll allow you to install Ubuntu on a separate partition.