I just bought a new magic tilt trailer. It has 8ft bunks in the middle and 4ft on the sides. The boat tries to roll to the side when loading on the trailer if you walk to the front. I think if I replace the 4ft bunks with 8's it would work better. Any advice?

Stop backing down so far. Run your boat up on the trailer.

Its not a depth of trailer in the water issue it is more of not enough bunk on the sides of boat issue. I was just curious if anyone else had/has similar issue.

If your boat is sitting properly on the trailer your dont need ANY side bunks. They are guides and not made to keep a boat on the trailer. If your boat is still float when its fully on the trailer it is to far in the water. I see this all the time at the launch. Alot of folks back down to far and fight trying to get back on the trailer.

But you could always add longer bunks or maybe raise them.

I am not talking about the side bunks or runners I am talking about the bunks the boat sits on.

So you have 4 runners on the trailer? Take the two 4' boards off and move the 8' boards out so that the boat sits on them properly.

My trailer has all 4 the same length bunks. Its a custom trailer, but I have 4 2x6x16's. The longer outside bunks make an easy place to walk if needed.
You can see the way mine is set up here.