It couldn't hurt to make the drill size the same as screw diameter or a tad larger as the screw head will hold the boards down. No need to put a lot of pressure on the wood decking side ways - this includes pre countersinking each hole. The screw head is a wedge shape head and can spilt the wood.
Gravity holds the deck boards in place and the screws hold them in location and stops side-to-side movement mostly with a little bit of down force from the screw.
You can buy drill bits with a counter sunk head built-in so drilling is a one step operation. Strictly time saving and a convenience but worth while on a huge deck. You can also use two drills like me one drill bit the other a counter sink rose. I have that gear configuration so I use it.
If you already have splits, I would squirt some Aquadhere Exterior from its bottle as deep into the cracks as possible then extract the offending screw allowing the crack to close. So if you use wedge shaped pegs you can gently place them into each side of the deck plank like a mini clamp pushing the crack closed more without having to unscrew the whole length. You may have to pull a couple pegs apart for the wedges if this works for you, or clamp planks closed gently for 24 hours. Once the glue has dried, re-drill hole and do a counter sinked hole and place screw back in holding the decking.
I would re-drill and countersink all of the screws installed to take that pressure of the boards as they are constantly expanding and contracting in the weather. This is a good use of you drill's torque control setting - a medium firm screw in, not a pile driver. - Jewelleryrescue
That's excellent recommendations coming from @Jewelleryrescue. Just a friendly reminder to measure your drilling distance when drilling into decking boards and that's 15mm from the edge of the board and 20mm from the top. Splitting often occurs when the pilot hole is too small and the screw is placed in too tight. When the board contracts it pulls on the screw and this constant push-and-pull cycle is what caused the timber to split. - EricL