How much do tides affect earth's rotational velocity?
This question is handeled pretty throughly in Physics site; 
https://physics.stackexchange.com/questions/214042/earths-kinetic-energy-change

Your question; "How big is the difference?"

Body Tides Summarize
0.3 ms for a 14 day period.
0.16 ms for a 28 day period
0.14 ms for 1/2 year period
0.02 ms for a year period.
0.126 ms for Lunar node period; (18.6 years)  

Ocean Tides
Max 0.05 ms for a 14 days period
0.026 ms for a Years period
0.024 ms for lunar node period