Scarleteen

What's the advantages/disadvantages of both girth and length? What's better? What can one do that the other can not?

I'm going to assume you're talking about girth & length in regards to penis size, here; if I'm wrong, please correct me!
There really isn't a "better" when it comes to any aspect of penis size or shape. Some people have preferences for a certain girth or length but many people either don't, or their preferences are very mild.
If someone does have a preference for the size or shape of a partner's penis, that preference might be influenced by factors like what kinds of sex they're interested in and what sensations they enjoy, so we can't say there's one universal thing that people prefer or find feels better.
And preferences aside, all that tells you is what one person likes; it isn't a value judgment on any one penis, if that makes sense.

What's the benefits of having a 'girthier' penis?

Hi TS_6249,

I think Mo has gone over this pretty well - there aren't any particular benefits, it all depends on what you like and what your partner likes. Some people like girthier penises, some people prefer less girthy penises. Same with length. Some people like all penises; some don't like any!

Can I ask where this question is coming from? Do you have a particular concern?

In regards to positions maybe. Do you have any advice on that?

Hey TS_6249,

Sorry to be a broken record, but the positions that work best and are comfiest are gonna depend on you and your partner(s). Communicating openly about when they are or aren't comfortable and what feels best to you both is going to be the best way to know what kinds of positions suit you.