First of all you need to make sure that different size wheel will fit. The first thing to check is that brakes still fit inside the wheel (obviously non-issue when going to bigger wheel size). Second thing is that wheel and tyre must not contact other parts during full in and out suspension, or turning the front wheels. This is a non-issue if you keep the width of the rim same, offset/ET (or backspacing which is same thing but measured differently) the same and outer diameter of the tyre the same. This would also eliminate the issues with engine revs and speedometer, that others pointed out.
Another consideration is, that if you're looking at offroad tyres you'd want to keep the profile of the tyre as high as possible, and thus the wheel size as small as possible (which still fits the brakes etc.) This allows using lower pressure, helps in tyre self-cleaning etc.
Looking at specs it seems you could have 245/65R17 as the OEM tyre/wheel size:
Full Specifications | Mitsubishi L200 | Mitsubishi Motors UK
You didn't specify what kind of conditions you're driving in, but for a quick no-brainer solution you could just order Goodyear All-Terrain tyres from Camskill and be done with it:
Goodyear Tyres / SUV 4x4 / Goodyear ATR Wrangler Goodyear AT R - 245/65 R17 107T (JE) TL Fuel Eff.: E Wet Grip: C NoiseClass: 3 Noise: 73dB £117.40 SUV Tyres - 4x4 Tyres - All Terrain Tyres - Off Road Tyres - 17" R17" - 245/65/17, 245/65R17
If you want to be serious first you need to do research which would be the conditions you'd be driving in and select appropriate tyres. Basically Mud Terrain if you're concerned about getting stuck on fields or similar, and All Terrain otherwise. A random internet search suggests there's 16" wheels available for your vehicle so I'd suggest taking that route:
Mitsubishi L200 4WD wheel sizes - Wheel size
Especially the 205/80R16 shows (same Camskill site as in above link) very good availability for different offroad tyres (is it used in some Land Rovers?) but also the 245/70R16 has some good alternatives like BF Goodrich All Terrain and the footprint is 20% larger. I wouldn't go over the nominal tyre sizes if you don't have suspension lift etc.