After watching this video about ImageRectOffset/Size he says that roblox decal is 256x256
Is there a way to make them bigger?
The limit is the maximum resolution. Anything higher than that resolution gets scaled down to the maximum resolution.
The maximum used to be 256x256, but now it is 1024x1024. Any image with larger dimensions than 1024x1024 will be scaled down.
There is no limit. It's just that a roblox decal is 256x256 when you publish it. When you put a decal on a part and you expand the size it only stretches out. You should note that the bigger it's stretched out the lesser the quality becomes.
Hope it helps!