Your path size could be 30" but you want to take into consideration your garden cart or wagons width,(that you move material in/out of your garden)as you will be adding soil,harvesting etc.
I made mine 42" as I'm in a manual wheelchair and wanted to be able to move a bit left or right when I had visitors in the garden, I forgot about the size of my gardencart so I asked for and recieved a smaller one for Christmas.
I used 2x12 which make them a nice height for me. They are 28 or 30" wide (cant remember exactly)which is also a good reaching width for me.
They are 20' long as I used 2 10'planks and did a lap joint with a poly glue to make 20'sides.
Cost was $10.per 2x12x10 planks of native green hemlock.
I put in a couple of flat metal fence post stakes cut in half on each side to keep the sides in place.
Hope this helps you out.LEt me know if you would like more info.

Jerome

Jerome,
That information is most helpful. I hadn't thought about a garden cart. My Home Depot has only cedar and redwood lumber to use for the sides. I think that will be ok to use. Thanks so much for helping me plan. If anyone else has any other thoughts I should consider, I'll be happy to hear from you, too.
Thanks. Sunny

Glad to have been of some help. A link for the type of cart I use(been over 18years of use)http://www.cartsvermont.com/

Ask around for a lumber yard that carrys local lumber.In your area I think Ponderosa Pine is a good local lumber.You want to buy it green (not dryed but freshly cut)
as that will keep the price down.
Ask the sawyer for the native pine(or other species) that will do well for your raised beds.
Items like this you do much bettter to deal with a local dealer and not the big box.
You may even find a local dealer that has cedar/redwood in the 2x12 size at a much better price. Again speak to local wood people in your area that know which type will be the best for you.

Hey Sunny. I used 2 & 3 liter recycled soda bottles. Head over to container gardening and I have a project going you might like to look at. It's under vegatables in container.