Generally, air "leakage" is not a problem. Drafts are a problem. Most resin sheds in factory condition have plenty of leakage (not a concern), no drafts to speak of at heights which are problems for your birds (roosting/nesting height), and essentially no ventilation. The first thing most do converting resin is cutting big holes in it, high up.
If you can see light, but not feel a breeze on a moderate wind day, you are fine, leave it alone. Rodents aren't a concern - they will just chew thru whatever sized hole they feel they need - a pencil width's visible light isn't the issue, the toilet paper roll sized hole is.
Post some pictures, we'll see if we can't offer recommends on where to locate ventilation. Also, your location (with as much specificity as you are willing to provide, so we have some idea of climate considerations), dimensions, planned flock size.