I know how to change it when the player mouses over the block, but how do I make the area they can put the mouse on larger than one block?

I don't think you can, if there is, I have never heard of it.

You can make the colideboundingboxes bigger. that is like a extendet block^^

i do not think that is a perfect result. But its a idea^^

The only real way to do it is with gag blocks (fake blocks which become "part" of your block)

 

There's various ways of doing it. Either use a TileEntity to store the locations, or just place and remove them at the same time as you place and remove the core block.

 

 

This wont make the bounding box "bigger", but it will mean players can't walk through your block.

I was hoping I wouldn't have to do that, but ok.

well yknow how you can make bb smaller like with slab... what happen if you make it bigger ?

that's what I tried. if i make it overlap into other blocks, it doesn't work. it doesn't allow you to put the mouse over a part of another block and work.

ok,  well the only other way i see is to spawn a custom entity at thsi location and try to make it have an outline when the player cursor hovers

 

but i think flenix way is even easier (or at least more straightfoward)