What size tires do you all have on your trailers? I have an older Sooner Dynasty 4H gooseneck, aluminum.. it has 215"s on it and someone I know was shocked that they are so small. Is this really that small? How much difference would bigger tires make? Maybe if I put 235's on?

Ideally I need my trailer to sit a few inches higher..my GN is nearly hitting my truck bed. Would new tires fix this issue??

You didn't state what rim size you had, But assuming 16"

A 215/85r16 tire is 30.4" tall

A 235/85r16 tire is 31.5" tall 

So changing the tire would raise your trailer 1/2 of this difference ( half of the difference is below the axle, half above the axle) Hence you would raise your trailer just over 1/2 inch off the ground by changing tires.

The big difference is in the load capacity.  The 215 tire are rated at 2680 lbs per tire, the 235 tire is rated at 3042 lbs per tire.  That's an increase of 362 lbs per tire. Times two tires = 724 lbs or times four = 1448 lbs increase in load.