Not knowing what you have available for cutting & joining sticks, some
For one, splitting and checking is generally more of a problem with
larger-section-dimension lumber. Really big stuff gets heavy, which
may be a +. Making clean cuts will be a challenge, unless you have one
of the real monster circ-saws, and can guide it.
OTOH, dimension lumber from most mills has rounded corners, making for
surface gaps when gluing. Getting such sticks aligned precisely, and
clamped evenly for gluing must be a real hoot. Laminations are
inherently difficult/impossible to split through.
Probably depends most on quality of what's available locally. Does
your supplier carry lumber better than stud to utility grade? Can you
choose pieces (like, no knots)?
And ... 4x6 for the top? To support big-block engines?
I've found that doubled 3/4" CDX plywood is extremely strong, making
excellent web for small beams.