The size of the ET will make a difference in how much the pressure rises, but since the water incompressible, with the ET precharge set higher than your static pressure, as soon as any expansion starts, it will immediately jump to 65-psi, your precharge value. Then, depending on how much expansion you get, the pressure will rise still…the bladder in the ET only compresses as the water expands, but that expansion of the water is compressing the bladder (balloon) into a smaller space, which means that it’s pressure must rise, too. The larger the ET, the smaller the percentage of the air is compressed for whatever expansion occurs in the water.
The reason it immediately drops to your PRV setting is, the water stored in the ET gets pushed out since it’s pressure is higher than your supply when you open a valve. If you have a slow leak anywhere, the pressure would slowly bleed off. The fact that it goes up indicates you don’t have a leak, which is a good thing!
So, since the amount of expansion depends on the volume of the WH you’re reheating, and when in the cycle of use, say using most of the tank, then stopping water use would create the largest volume, and thus, the largest pressure rise. Washing your hands or maybe a load of dishes in the DW won’t use anywhere near as much hot water as the average person’s shower, and some will empty their WH, which will lead to the worst case.
FWIW, the only ‘cost’ to a larger ET is a small incremental cost to buy it, and the support and size of it when installing it in prep for when it fails the next time…larger tank, larger volume, less pressure rise, more weight to hold up without stressing the plumbing.