AI want to try using straws to support my cake tiers rather than wooden dowels. Should I use the milkshake straws (1/4 inch in diameter) or tea/boba straws (approximately 3/4 in diameter)? Thanks for your feedback!!

The general rule of thumb would be the larger the bore/hole, the better.  You get the most contact with the board that way, largest cake plug [acts like glue & a weight], etc. 

If the boba tea straws are readily available, I'd use those.

I think a lot has to do with the size of the tiers.  I always used milkshake straws.  Worked just fine for me.