Is the 6' tug and toss the right size for my Golden Retriever?

I do think this is the correct size for a Golden Retriever. I use if for my border collie. It is about the size of a soccer ball. It is a heavy plastic, so it helps wears her out to carry it around while fetching and walking. I kind of have to spin in a slight circle (like an Olympic discus thrower) to really get some distance when I toss the ball by hand. This is a good size for most the mid-sized to larger dogs. I think my border collie is around the 50 to 60 pound range now, and my German Shepard loved these balls and she was 65 to 70 pounds. Eventually our dogs have always chewed off the handles (it normally takes many months) because they love how easy it is to pick up and tote around by the handle, and it is large enough to wrestle it around and attack the handle. So the handles tend to attract some abuse. Yet, even when the handle wears off, the dogs can fit their teeth into the hole to pick up the ball easily, and I end up putting my fingers into it, to be able to throw the ball well by hand. Also, this ball holds up really well outdoors all the time, even in sub zero weather. The only thing to remember is, if you go to kick this ball in the middle of winter, the plastic/rubber gets harder in cold weather, and will hurt your foot if you kick it really hard - expecting the same flexibility.

what color or size (in inches) is the medium?

It's red and the 6 inch about the size of a cantaloupe. The 4 inch is about the size of a softball.

Are these hard plastic or a bit more squishy? I worry about my dog chipping a tooth if they are real solid

I go it for a German Shepherd (1.5 yrs old). She plays with it for hours at a time. The handle is not solid plastic, but its not thin (squishy). I think your safe buying it. I also have the 8 inch, I wanted something bigger for her. she's been playing with the 8 inch for 5 months, the dog and ball are both doing good. Now, if I was to give one to my yellow lab, I think he would chew the handle off, but the material would not hurt his teeth.