Quite easy to check on size of PSU, it is on a large label on the PSU, unfortunately (and I don't know why) but the label is usually on the top of the PSU making it impossible to see without at least partly removing the PSU.
Do you feel able to do this yourself? I can explain further if you wish.
Right, it will mean first, close down pc in normal way and then disconnect from the Mains.
If the sides of the Tower case are individually removable, take off the left-hand side (looking at pc from the Front) there will be two screws at back of case on the side you need to take off.
Next simply touch any metal part of the case in order to discharge any static electricity in your lovely body (don't worry, we all have static electricity in our bodies) - this is only a precaution to prevent static electricity from damaging internal components.
Then look at back of pc and you will see a metal grill with the Mains plug attached to the pc - this plug is actually attached to the PSU. So now you know where the PSU is you can locate four screws on outside of pc holding PSU to case (one on each corner of PSU) remove these screws and gently support weight of PSU and remove sideways from case - just enough to be able to see label on top of PSU. Make a note of Power it will be marked 350W or whatever.
Replace psu, tighten all screws, replace side of pc.
Re-insert Mains cable to PSU and re-start pc.
I will post this response now and check my instructions and give you time to read and if necessary make notes.