Button size increase, while other buttons slide over to allow space?

I have a line of 12 horizontal placed buttons. I want whatever button is rolled over to get larger (no problem), but I need the other buttons to slide over to allow for the new size that the larger one would bring. I have no idea what type of code I should be looking for here.

Can anyone lead me in some sort of direction or drop a hint?

thanks for any info!!!
mx 2004