The size of the adapter depends on the size of the filters and any other accessory you wish to attach to it. I happen to have two 58mm filters left over from my 35mm SLR days so I bought the 58mm lens adapter. Otherwise I probably would have bought the 52mm size.
Think about filters and converter lenses you may possibly buy in the future. For example, if you want to buy a particular lens, check the size of it's mounting threads.
Some photographers buy two adapters, one of each size, depending on what accessories they have.
Go back and look here a bit closer at what's available and the sizes of the treads.
http://www.lensmateonline.com/newsite/order_S2_1.htmlIf you just want a few filters, such as I do, make sure the filters you buy have the same size threads as the adapter.
Lensmate is very helpful. Here's their email address if you have specific questions about their products.
susan.sales@lensmateonline.comAlso let us know if you have other questions.
Related reading - converter lenseshttp://www.digicamhelp.com/digital-camera-converter-lenses/index.htm