irons

What are the advantages and disadvantages of having oversized club heads on irons over the standard size? And about the flex for irons: For the stiff, is the loss of distance worth the gain in accuracy or vice versa for the regular flex?

Oversized clubfaces are supposed to be more forgiving, meaning that off-centre shots won't turn out as bad as they should with regular-sized clubfaces.

Personally I like regular-size more as I can shape the shot a lot better. A beginner will likely be suited better with oversized.

As for regular or stiff shaft, I think that's more of a personal preference. I started using a regular flex driver. As I developed my swing over the years (3 years), I also felt more confident with my game. My swing speed increased but not too much. Now I play a stiff shaft and it feels more comfortable.

Go out and try a few before making your decision. It's hard to explain to you in writing about "the feel". You really have to experience it first-hand.