Is a 30GB SSD large enough to install Windows 7 Professional on?

No, its not. You will want at least 60gig to install Windows, and a few of your basic programs on, just a few. 120gig is recommended.

I wouldn't even consider a 60GB, The OS itself doesn't take that much space, but you need to have space for the OS continuous upgrades, a few programs, and above all the bigger the SSD the faster it will be.

Note: I still don't understand why this is true bigger=faster, but it is.

Yes, it is. In this case it is very odd with SSD's but understandable when you consider how NAND chips operate.

It's funny because we have contradicting sayings where Bigger is Better and then you have Less is More.

With advancements in technology most of us won't have physical computers anymore.

It's actually a bit daunting that in my school our IT classes are utilizing VMWare for a majority of the programs we use.
As much as I hate it the cloud is the future. Aim for the skies!

While that is all very good advice, I have had Windows 7 installed on a 60gig SSD with a few other programs for about 18 months now. I am currently at 35gig used on my 60gig SSD. 60gig IS plenty big enough for the OS, Antivirus, your browser, Microsoft Office, and a few other basic things. But that is all it is big enough for, and any data storage or other major program installs is out of the question. That is why I suggest 120gig.

Windows 7 64 bit requires a minimum of 20 Gig and 32 bit Windows 7 requires 16 Gig. If you get a 30 Gig SSD you will be ever lastingly juggling your files to free up space. Also a 30 Gig SSD will be slow I recommend that you get a 120Gig SSD it will be faster and more convenient.

I suggest you'd better get at least 64GB SSD for system installing, as we know Windows 7 64 BIT os required at least 20GB free space and 32bit required 16GB. However, if you take 30 GB ssd for OS, it will be not enough.

Eh? I run a 64GB Samsung with Win 7 Pro 64bit, MS Office (Word, Excel, PP), MS Outlook 2012, MS Expression Web, Photoshop CS5 (both the 32 and 64bit version), Vegas 11 Pro, Norton IS 2012, Firefox, Skype, Filezilla and a bunch of filters and other smaller stuff... and still have 36 GB to spare.

So what is your point here? Isn't that pretty much EXACTLY what I just said? Now, start storing data on it, you know, some movies, some videos, some pictures, your backups, all the stuff that most normal people store, and see how long it lasts. I have over 20 gig of just music alone.

30 Gigs - NO. A 30 gig is really only about 25 Gigs of USABLE space. You lose 2 gigs in way manufs count and then another 3 gigs as you MUST leave UNUSED for wear leveling, CG and Trim to work their magic.

60/64 Gigs is Normally considered the Minimum. Personally I recommend 80 as the Min an the recommended as 120/128 gigs. With 60 gigs, you lose about 10 gigs off the top so that's down to 50 gigs usable. Have 3 Systems, OS + programs, NO GAMES - NO User generated data, and all use between 35->40 gigs. AND that is after: disabling hibernation, limiting page file to 1024 mb, and disabling restore points.

ADDED:
On performance vs size. Normally internally drives over 60 gigs use an internal raid0 config. For instant a 60 gig dive is like a single drive, but a 120 gig drive (internally) is two 60 gig drives in a Raid0 config. Raid0 provides a big boost to sequencial performance and a minor boost to Random read/writes.
.. Then there is longevity. EX say you have a 60 gig SSD with 35 gigs on it, now you start writing to the drive, your writes will be spread over 25 gigs (were leveling sill spread the writes around, and even move files that you only read). With a larger drive ther writes are spread out over a larger area thus less writes per cell.