48x48 is the maximum standard size. There is no one, single "standard" pallet size - there are several standards as defined by the ISO.
The most common pallet size in North America is the GMA (Grocery Manufacturers' Association) pallet, which is 40x48, and accounts for more than 30% of the half a billion new wood pallets produced for use in North America each year.
Next up is 42x42, and then 48x48.
Other common sizes are 44x44, 36x36, and 36x48.
In Automotive, the most common is 45x48 and 48x48. For drums, a standard 55-gallon drum is 22.5 inches in diameter (33.5 inches tall), you see a lot of 45x45 pallets used for those.
Incidentally, a 55-gallon drum holds 55 US gallons, which is 46 Imperial gallons, yet the Brits (and many in Canada) refer to them as 44 gallon drums. No one knows why. One barrel of crude oil is 42 gallons.
Also, just as a matter of note, random samplings of pallets regularly show that 10% of wooden pallets and 1.4% of plastic pallets will test positive for as E. coli and/or Listeria. So wash your hands after handling a pallet, and for certain don't be lickin' pallets.