Size is one way to tell them apart, but if you look closely, you can see that the coloration and pattern on the dorsal (back) side is different as well.
I have contracted Lyme Disease at least 6 times over the course of my tramping around places like Rondeau for more than 40 years. I posted a fairly extensive blog post about it a year ago, and you can read about it here.
I made a follow-up post a little later, which is shown here.
In some jurisdictions, May is Lyme Disease Awareness Month. To learn more about what the Canadian Lyme Disease Foundation has to offer in terms of science, awareness and support, check out their link.
Enjoy the outdoors, but be safe!