There is definitely a technique required for surfacing the flat side of a board. It's not a 1 pass operation and you have a flat surface.
Here's what I do. I sight down the surface and see if it's flat , concave or convex. I like to put the concave surface down first so it touches on two points at either end. Then I "partially" plane the concave ends alternately flipping it end for end by just planing in far enough and repeatedly until it looks flat to my sight. I'm trying to remove the wood that is not part of a flat surface.
When It's relatively flat then I will run the entire length of the board over the cutter until the color is even and it will not rock on a large flat surface, still sighting it by eye.
Then that large flat surface is placed against the fence and the first edge is jointed square to it. If the board is curved, you can plane in partially, removing the material which is concave, sighting each time and flipping end for end as necessary.
A 6 ft board is about all I want to joint on a 6" jointer, but I have used roller supports for longer ones. You have to be dead on with your support height or you will not get a straight or flat surface.