Bicycle Mechanics - Adjustments for larger chainring?

Any issues (Derailluer, adjustments, chain length) with changing to a larger chainring on a 10 year old GT mountain bike? The current chainring is a 42 tooth Shimano with a label that says "Shimano IG chain only", the front derailluer is Shimano Alvio and the rear is Shimano STX.

I have a flat commute and I max out the current gearing. What's the biggest chainring I can use?

It's impossible to say without seeing your bike. Here are the potential issues:

1. Possible interference with the chainstay.
2. Most mountain bike front derailleurs are rated for up to 46t or 48t chainrings. Bigger than that and you will have to mount the derailleur so high that it will degrade the shifting. 50t and up you'll probably need a road front derailleur which will require under the bottom bracket cable routing.
3. Front shifting may be degraded anyway if the difference between the middle ring and big chainring is too big.
4. You'll have to raise your front derailleur for sure.
5. You'll have to lengthen your chain.

You'd have more luck and achieve the same result by changing your cassette (rear) gearing.

You'd have more luck and achieve the same result by changing your cassette (rear) gearing.

If he has a 42t big ring he probably also has an 11t small rear cog. Not gonna be able to improve on that.

If he has a 42t big ring he probably also has an 11t small rear cog. Not gonna be able to improve on that.

Yes, I have an 11 tooth small cog on the rear cluster.