1) Larger SSD's perform better than smaller ones. That is because internally, they can access more of the nand storage chips concurrently. Sort of an internal raid-0. No need for raid of ssd devices. Also, raid controllers currently do not pass the trim command to the ssd. That will cause ssd performance to degrade as the ssd gets filled. In order for windows-7 to transmit the trim command, the sata mode in the bios must be set to AHCI(not IDE, or RAID).
2) Windows 7 will take about 13gb withought any attemp to reduce it's size.
3) raid-0,aka striping, is helpful for large block sequential operations, not much else. The OS does mostly small random operations.
4) raid-1 duplicates data on two different drives. The value of raid-1 and it's variants like raid-5 for protecting data is that you can recover from a hard drive failure quickly.
It is for servers that can't afford any down time.
Recovery from a hard drive failure is just moments.
Fortunately hard drives do not fail often.
Mean time to failure is claimed to be on the order of 1,000,000 hours.(100 years)
Raid-1 does not protect you from other types of losses such as viruses,
software errors,raid controller failure, operator error, or fire...etc.
For that, you need EXTERNAL backup.
If you have external backup, and can afford some recovery time, then you don't need raid-1.