Currently I’m due for new tires on my 2021 ascent Premium and it currently have from stock 245/60r18 continental tires that I drove at 26,352 mileage. I’m from south Florida. Don’t want to get the same tire since it doesn’t seem to last long. I am looking at Yokohama Geolanders but noticed they sale them at a larger size. How much bigger can I go with the original rims? For example 265/60r18 or 265/55r18? Any suggestions?
I don’t think the sizes you list will fit. Maybe 255/65. Look at Discount Tire; they list alternate sizes that fit.
But just going to a larger tire won’t necessarily give you longer tread life. And could have unintended consequences.
Stock tires typically ride soft and cushy, but don’t have great tread wear. Going to a higher quality tire might accomplish your objective. I believe both the Michelin Defender LTX and Continental Terraincontact HT come in your size and should last quite a bit longer.
Cheers!
Why do you want bigger tires?
The 265mm tire is .787 inches (or just under 4/5 inches) wider than the 245's, or just under 2/5 inch wider on each side from midline (center of the tire tread). If that width difference fits the wheel well including the up and down travel distance of the shocks/struts, you should be okay.
The 245/60 has a profile height of 147mm (5.787 inches) while the 265/55 has a profile height of 145.75mm (5.738 inches). A decrease of 1.25 mm or .05 inches in radius which would result in a .31 inch decrease in tire circumference. So at 60mph using the 245/60 tires, the speedometer would be reading 59.81mph using the 265/55's. Not much of a difference.
But, I have been up for the past 19 hours today so you might want to check my computations. 
Additionally, your local tire store should be able to provide information regarding alternative tire sizes to fit your vehicle.