Would this help?

 

1/2 the width + 1/32~1/16"

 

Some manufacturers advertise certain of their cutters as being able to make slots for biscuits so just check their websites to see which ones are recommended. The big drawback of using a router for making biscuit slots is that you can only make them near an edge.

 

To follow up on the suggestion made by @Cherryville Chuck here is a link to one router bit seller web site slot cutter page that lists which cutters are suited for which size biscuits: A 5/32" carbide height/width of cut/kerf, 9/16" deep cut is what is listed for #20 (and #10 and #0) biscuits.

MLCS Slot Cutter Router Bits and Stackable Slot Cutter Set

Although I am far from expert on this topic (read "I know what a biscuit is") having read enough posts by @Stick486 I wonder if splines would not be better.

 

JMHO, splines nor biscuits provide much more than an alignment aid. Except in a very few instances do they add a measurable amount of strength to a glued joint.
Again, JMHO, they are a waste of time.
YMMV

 

sorry you feel that way about splines...

 

So, educate me about those applications where they are useful.
Hey, I'm still larnin'.

 

door and table panels... most any edge or end grain glue up...

 

Haven't built any doors, but I can see where it might be beneficial. As to edge grain glue ups, Ive done a bunch without splines. None have failed.
Made splines on breadboard ends but, those are integral to the joint.
End grain to edge or face grain is usually an M&T. Pocket screws, if it doesn't show.

 

consider a spline a floating stub M&T joint...

 

A spline is a continuous floating tongue in a tongue and double groove joint.

Biscuits are interrupted splines in double interrupted grooves.

Both are versions of a mortise and tenon joint, the mortise and tenon being typically much longer and deeper.

The strength of all 3 is that the wooden spline/biscuit/tenon provides a strong mechanical barrier to vertical shear forces as those forces have to shear the wood and the glue, In a butt joint the glue would have 2 planes of shear failure, both horizontally and vertically. Biscuit/spline/mortise joints have mostly only one shear plane which is parallel to the face of the biscuit/spline/tenon or horizontal shear. Obviously, the longer this plane is, the greater the resistance to shear failure. That's the engineering explanation.

 

OK,, I gotcha, now.

 

splines can be adjusted to any thickness and length you wish...
use the 1/3rd rule.. 1/4 spline for 3/4 material, 2/4 for 6/4 and so on...
this where biscuits fall way short...
perpendicular grained splines in RO make for some serious strength... and there is that extended widths of the splines that's a plus..

heat causes PVA glues to plasticize when exposed to heat...
remove the heat in the early stages of plasticization and the glue resets as it was..
but in the process, plasticized glue's strength ''slips'' and you can experience joint creep (not flush/aligned anymore) separation or even total failure...
heat gun PVA joints.. they come apart..
take your dried PVA glue brush and run it under hot tap water...
the glue melts and washes away and your brush becomes good as new...
take that glued up panel w/o splines and leave it your truck on a sunny day and it will come apart or the seams will at least creep/separate some...
your finished table is sitting the sun coming through the picture window for and extended period.. same deal...
the top can go a little or a lot wonky..
presto... unhappy campers and customers.. who needs the rework...
splines will/can hedge your bets in your favor...
splines done w/ a little fore thought can give you flatter tops (less planing/sanding) and less prone to joint curl....
VOE...

 

An important thing to remember when slotting for biscuits is to slide the jointer slightly left and right whilst fully plunged, if slotting on the router table the same applies. The idea of this is to allow sideways movement for perfect alignment.
When jointing long boards as for table tops, providing one has the equipment to make long splines, it's an excellent way to go.

 

When I made my fireplace mantel the top needed to be 14" wide and 14" wide material is hard to come by so I had to join a 2x8 and 2x6 to get that. Being over a wood burning fireplace insert that goes 24/7 during the winter I was a little concerned about wood movement so I thought I had better add a spline between them so I cut a 1/2" wide groove in each board on a table saw. It took less than 5 minutes. I set a gap between the blade and fence that was roughly half of the thickness of the boards and made a pass and then flipped the board around and passed it though the other way so that the groove stayed centered on each board. A handheld router could have done the same job by working from each face in the same fashion. 20 years later the glue up is still holding just fine.

On another note, when I glued it up it took a few minutes to get the glue on as the boards were over 7 feet long and the glue had started to skin a bit by the time I got ready to join them which made them very hard to pull together. I had to use pipe clamps to close the gaps which did work but just barely. So if you are going to take a while to glue the joint up you have to make sure the glue you are using has a long enough open time to get it done.