I want to mount up bigger wheels
This is not a common process and certainly is not plug and play. Motorcycle wheels don't easily interchange like car wheels. This is usually only done by racers who are looking for wider wheels so they can run radial tires.
Anyone interested in going to 17" wheels for a better selection of rubber should get a set of 16" wheels welded up by Kosman. This will change the wheel sizes to 3.25-17" front and 4.50-17" rear, which will fit 150-width radial tires out back and 110 on the front. This runs about $1200, and then you'll want to run better, more expensive tires, too.
Unless you're going to race it's not worth it. Even if you are going to race, it's still probably not worth it. :-)
Kosman wheels at left, stockers right.
Sarcastic answer for those doing this for cosmetic reasons
There isn't really enough clearance in the swing arm for a really big tire to fit, unless you get the wheels professionally done up (see above). That isn't going to stop some people, though. So...
Buy EX650 front and rear wheels on eBay. Buy brake disks, calipers, master cylinders, lines, and probably a whole host of bushings and carriers. Then you will need about 4-6 hours of mill/lathe time; a welder would also be helpful. You will have to mill the 650 rear wheel to fit inside the swing arm, then mill the swing arm to fit the wheel. After that you will have to reinforce the arm because you just removed a lot of material.
After fighting with the sprocket carrier and brake for some time, you will now move to the front end. Here you will fight with fitting a much larger wheel than the forks were designed for. The number of bushings that will need to be used will have to be determined as you use them.
After a while you will realize that you should have just purchased the swingarm and forks from a 650 and mounted them (with the same amount of work) but then you think, "Wait, why would so many engineers put small wheels on small bikes". Then you would actually read a little, and maybe a little more.
At this point your bike is in a lot of pieces and you decide to go buy a gixR1636 because you have ridden your 250 for 2 months/200 miles and you got it to "learn on", anyway.