Welcome to the forums!
I'm far from a monitor expert, so a good idea to check my information with more people.
Unfortunately, my memory of the G75VW is a bit fuzzy at this point, but guessing you can attach two external monitors although it may be necessary to disable the internal notebook display if the vga card can only support 2 displays total, so don't forget to try this if necessary.
From what little I know about monitors it depends more on the resolution you are running the monitors at rather than the size of the monitor display itself although they are often related since bigger displays often run at higher resolutions.
If you run at a higher resolution, your vga card may not support higher monitor refresh rates, so something else to watch out for.
Personally for me with my poor eyesight, I usually try to get the lowest resolution on the biggest display available to avoid straining my eyes all the time .
Also, don't know if you will plan to play games, run Photoshop or do anything more 'graphically intensive', but how you plan to use the system makes a difference too if your graphics card will be able to handle the loading since writing code or doing spreadsheets isn't going to put as much of a strain on your graphics card as other things.
Finally, if you weren't aware of it, the G75VW appears to support HDMI, mini-display port, as well as analog vga monitor output and using the analog vga output should provide the poorest quality output of the three, so encourage you to use the other two outputs.
It's probably difficult, but if you can hook up your system to one or two monitors to 'test' and make sure you don't see any problems, it would be a good idea and maybe you have a local store willing to help with this although you can also try to dig up specs or old posts related to this.
Although it may be possible to replace the G75VW vga card module, ASUS has not provided any easy and convenient upgrade paths for consumers to do this, so expect it would be very difficult to do any maybe not even worth the effort in my opinion although you may be able to find some old threads about it online and in the ROG forums if you want to pursue it on your own.