A couple points not addressed in this thread are... Larger fuel tanks for street legal bikes are virtually non existent in the aftermarket, even for carbureted bikes, let alone motorcycles equipped with EFI. A larger tank poses a problem on any motorcycle, in that the additional weight when full is located high on the bike, thereby raising the center of gravity, or mass, which then will affect the handling of the bike. On any modern bike the size and location of the fuel tank is an integral part of the design process, and is a key consideration with regard to vehicle handling dynamics. That said, countless older, carb equipped street bikes have had larger fuel tanks retrofitted, as it can be relatively easy to do as long as the bigger tank fits between the steering head and seat, and doesn't interfere with the forks. Fuel tanks on carbureted bikes generally all have a fuel petcock, or shutoff valve, so the connection between fuel tank and carb is simple... "Plug and play". This is where the big difference comes in with regard to bikes equipped with EFI. The EFI system requires a fuel pump, which is located in the fuel tank, to operate. An EFI fuel system is a "closed loop" system... fuel that is pumped to the throttle body/injector, which is not "used" by the injector to feed the engine, is returned to the fuel tank. Carbureted bikes rely on gravity to feed fuel to the carb.
Going back to the CBR250R, lets say that we are going to redesign the fuel tank, which we could (as long as cost is no object). The only areas to practically increase the capacity is to make the new tank wider, which will affect ergonomics, or make it taller, affecting CG. Obviously honda has already made the tank to sit as low as possible in the frame.
In the end we would not be able to increase capacity to any large degree, regardless of how well funded our project is. This is, in a nutshell, why no aftermarket company would undertake such an endeavor... production cost would be very high. Would you pay $300.00 (likely much more), for a new fuel tank that realistically may only be 1/2 a gallon larger? Didn't think so. Large tanks are made for off road bikes, some as large as 4 to 6 gallons. They are plastic, and are not cheap. Street legal motorcycles sold new in the United States are required to have fuel tanks made of steel, off road bikes do not have to meet that requirement.
If you did come up with an auxiliary tank for the passenger seat area, or hang it off the back of the bike as shown in the previous post, how would you practically integrate it into the closed EFI system? As someone who has a degree in Mechanical Engineering, I can't think of any way to do it. Not with just a few feet of fuel line anyway, and certainly not without a large pile of cash.
Of all the previous posts on this thread, the idea of carrying a couple 500ml fuel bottles, which can be had for about $12 at REI is the best, lowest cost solution I can think of.