Can I put a 700c (28") wheel on a 26" or 650b fork, it looks like I'll have plenty of room but I would need to get something to move my v brakes as they wouldn't touch the rim
I assume you have a mountain bike, as you have a 26"/27.5" fork.
Answering the question in the title:
26", 27.5" and 29" mountain bike wheels all have approximately that diameter with a tire installed. Obviously the actual diameter of a particular wheel depends on the size of tire installed on it.
The rim sizes are 559, 584 and 622mm respectively, as defined by ISO standards. In road bike circles 584 and 622mm rims are also known as 650b and 700c - hangovers from a old French wheel size system.
So on a MTB, you need about 1.5" of extra clearance to fit a 29" wheel instead of a 26" wheel.
Can you fit a 29" wheel on your 26"/27.5" fork? Not really.
Even if you have the clearance, adapters to move the rim brakes are not available (to my knowledge, maybe there is some speciality manufacturer out there).
In addition, bike frame and fork geometry is designed around a specific wheel diameter. If you change the wheel size, the steering geometry will be altered, usually with negative effects.
If you are comfortable riding a frankenbike, there are several scenarios where you can put a 622 wheel in a 26" fork. The brake contact location moves about 32mm, but if you run disc or hub brakes they will not be impacted, and v-brake arm lengths vary from 75 mm to 120 mm, so if the mounting points on your fork are not too low you can switch out v-brake arms.
If you keep a similar final tire diameter, the fork should not have clearance issues and the geometry and stresses on the frame will remain the same. So, if you had 26"x2.50" to start with, a 622x32 mm would have the same final diameter. You could find an appropriate cyclocross tire in that size that would be reasonable on a 26" MTB frame. MTB frames also usually have extra clearance, so you might be able to squeeze in a 38 or 44 with slight changes to riding geometry.
You might also have to contend with different hub widths. 100, 110, and 120 mm are all available MTB front hub sizes, with 100 the most common, but if you're getting a free wheel it might be a different size. Steel forks can usually be gently bent in or out to accommodate different sizes of hub, but aluminum and composite forks should not be run with different size hubs. Even if you can wedge it in it can lead to failure when riding.
Even if you get it to fit, having different sized front and rear wheels will lead to challenges in maintenance, you'll need 2 spare tubes and 2 spare tires, you'll have different brake arm lengths that may apply braking force differently, and it will look funny. But if you can't afford to buy the correct replacement wheel and tire and need transportation, it will get you around.
Two great answers. A supplement to both is that everything will be higher including your bottom bracket, seatpost, toptube, and head tube (assuming you swap both front and rear):
Seatpost means that the minimum height of the seatpost will be higher.
Top tube means your top tube clearance will be less and your boy- or girl-bits may be at risk when making a planned or unplanned dismount
Bottom bracket means on the positive that you have more clearance but on the negative you may have to raise your seatpost higher to achieve optimal cranking and this may make you top heavy
Head tube means your handlebars will also be raised and this may limit your tuck-ability
If you swap just the front, your bike will feel as though it’s always going uphill.