How much can you increase a toys size?

Hello,

Recently started anal play and was wondering in what increment of size is a safe enough to challenge a bigger toy.

Currently I am capable of taking 6.5 inches and want to go bigger but don't know by how much is a good idea because when I had bought my dildo, I forgot to read the insertable length and only looked at the total length so I had only increase my toy size by an inch.

Didn't feel hard at all when increasing by an inch so is at least 2 the minimum/safest and 3 to 4 might be challenging but possible? And anything after that would just hurt if I made a jump in size that big?

Many thanks