Stock X6M 2013 came with 275/40/20 front and 315/35/20 rear. Recommended largest size is a 22" rim with 295/30/22 front and 335/25/22 rear as max tire size. This is the set up I run on my 2013 X6M. This keeps the overall rolling circumference the same as factory. If you went to a 24" rim tires size would need to be the same or as close to stock rolling circumference. I would think you are going to
greatly sacrifice ride quality going to a 24" 255/30/24 you are losing 2" of the sidewall so your sidewall will be approx 2.5" compared to 4.3" stock. That size tire will give you 1.2% difference in circumference so not a big deal. Fitment will depend on wheel offset as well. You can go tacomaworld and play with the tire calculator to see the difference.
https://www.tacomaworld.com/tirecalc...0r24-275-25r24