You would want to use a #6 or 7 for flattening.
The way to flatten a large panel is start out going 45° across the grain in one direction, then the other, then straight across. Using a straight edge and winding sticks to check for flatness and twist. I often just use the corner of the plane sole.
Once you're satisifed, you can move to final planing/sanding.
A word of caution: I do not recommend smooth planing the top unless you've oriented all the boards grain direction the same otherwise you'll probably be dealing with tearout.
In your situation, I suggest skipping the smooth plane & going straight to sanding.
Whole 'nother topic, but Grizzly hand planes are not a recognized quality brand. If you want new, IMO WoodRiver is an excellent value, or you can try finding an older Stanley.