The lower bunks on my trailer need to be replaced. They appear to be about 8' long but the longest ones carried by BPS are 6' long. The question I have is do they really have to be that long or will the 6' ones be okay. It doesn't look like the extra length will make much difference.

I put 6' on mine and had no problem. Mine is a 19' boat though.

When you cover them make sure you leave a gap on the bottom to allow drainage, otherwise the felt or carpet will hold water and rot quicker. Just a tip I got from a marine shop.

You can buy the carpet and lumber separately and make them the same size as the ones you are replacing.  I would not want to find out later that you really did need 8' bunks.  FYI, my 19' boat has 2 - 12' bunks and 2 - 8' bunks underneath.

Do you use marine grade carpeting or will inddor/outdoor carpeting work as well?

Go to Academy, BPS or any other marine retailer... they sell the carpet in pre cut (to fit 2x4 or 2x6 bunks) in several lengths.  Around here it's in 8' or 12' strips (you can half it for 4' & 6' bunks)  

It's probably cheaper to buy it that way than to try and make other types of carpet work. (easier, at least)  Academy sells the 12' roll for $12.  

I just replaced my trailer bunks w/ ones that i made out of lumber and indoor/outdoor carpet.  Two 7ft bunks and two 4' bunks for my Nitro.  total investment for it was roughly $20 altough it cost more than that but i didn't use all of the materials so im estimating that i used $20.  Very easy to do.  Took 2hrs to cut the wood to length, cut the right amount of carpet, and to glue/staple carpet to the wood.  And that time was for someone who had nver done it before.

I bought my bunk carpet from BPS and 2X4s from Home Depot.  Like KVD said its real easy...just a couple of hours to put it all together on the boat.   One tip....use stainless steel staples with the carpet glue to attach the carpet to the bunks.