How long are the screws that come with it?

It's been a while since I installed the opener but from what I remember I think the screws were only about a 1/4 ' long. It you're wondering if they are long enough to secure the opener they are more than long enough to do the job plus there is double stick adhesive tape to help keep it in place.

What is the largest lid diameter this will open?

I can't tell you in inches, but I opened a lid that was REALLY large - a larger jar from Sam's club. It was too large to open on either of my older openers.

Will this work with Mason jar lids?

Yes it will work on mason jar lids as well but if you push the jar too hard it will leave nicks on the lids and you could scratch ourself. You might want to use a good silicone jar opener if you love your mason jar lids to use over and over again like I do. Other then that my stainless steel jar opener is the best for everything else. I am so glad I bought it.

Hi - can you possibly send some replacement screws? one of the screws was stripped. i would appreciate a few. thanks!

I have not needed to use the screws at all. The adhesive is sufficient especially if you install with the V angle facing toward you towards the front and under the cabinet. If you need screw replacement a hardware store (not a big box store) is your best bet.