one way is to find the originals in the folder "widgets" - and replace them with your new icons. A good idea though, is to keep the old icons in a new folder fx called 'archive'.
Make sure the new ones are made as the same file type and with the exact same name as the ones you remove, and they should simply just be replaced
hmm mine are only 35x35 but I also use another skin - I think it's up to you to decide tbh. If you want them bigger of course, than the default, you need to make sure they still 'look good' and fit in the list.
Are you making new in Photoshop or ill, just test by importing the old one and check out the size and how the picture looks there