The size of photo that you send is all a matter of preference.
If you are printing the photo off your computer, then I would suggest using a small size photo as it requires a lot less ink. The image quality will also be greater if you size down a large photo rather than enlarging a small photo.
With almost every ink cartridge now selling for about $30.00+, smaller photos are also cheaper to use when you think about it.
I personally use 6x4 in (10x15 cm) paper. It takes less ink and is relatively easy to set up and print from Microsoft Word, etc.
However, if you choose to use 8x10s there are a number of ways to store the autos in.
You could use an 8x10 photo frame. This is a common frame size, and you should be able to find them wherever frames are sold.
You could also do as I do, and put them in an album. For my album, I use a three ring binder, and some photo sleeves made for a three ring binder. The photo sleeves are 8.5x11 so 8x10's don't fit perfect, but the work well. These sleeves also come in sizes that hold 6x4 and 5x7 as well.
If you choose to use a 6x9 photo, you will have to use an 8x10 frame and you will have to center it. This can be done by rolling a piece of tape up, and attaching to the back of the photo.
The album pages will also hold 6x9 photos, the photo just won't be a perfect fit.
if i send out a pic i use 175x125mm for size, but i get them printed off a website for a photo devolping company and upload them as digital photos, that means i dont have to pay for ink at all, its only about 10p a pictue plus 99p postage which is nice and cheap so id reccomend that to anyone!
i store my pics in an a4 display folder, instead of having to put in punched pockets it already has them in it so thats easier. plus just about everything should fit in. ive finally found some mounting things in wh smith so im going to mount what ive got on a4 paper so they dont slide around in there