I live in a one story rancher in New Jersey. I am looking to replace my front window with a bay window. Currently the front window is two double hung totaling 55 inches wide. There is a double 2x6 as a header now. The bay window is 84 inches wide. Can I use a double 2x8 or does it have to be bigger.

You may need a permit for this so check on that first. Local codes may dictate header size. Is this a load bearing wall or end wall?

I know I need a permit in NJ to change the header size but not sure what the code is. It is also a load bearing wall.

We need to know the width of your house in order to determine the header size.

'Width' is the outside wall-to-wall distance in the direction of your rafters and perpendicular to the ridge beam.

(The longer the rafters and ceiling joists the more load is placed on the header)

It is then a simple matter of looking in the IRC Code Tables for header sizes on exterior bearing walls.

Right now, it appears you are going to need at least 2 2x10s if your building width is 20 feet or less and more likley 3 2x10s for this 7ft header if your building width exceeds 20 feet.

Alternately you can install an LVL if header depth is an issue.

NJ uses the 2006 International Residential Code statewide.

The IRC also requires TWO jack studs under each end of this header whatever type you use, so you may end up having to increase the opening size in order to install it.