What does that mean? Should the yellow area become taller as text is added or wider as text is added? How much should it be allowed to change?
Did you read the following resources as requested?
(1) Writing the txt both vertically and horizentally centered - #2 by ronpat
(2) https://www.sitepoint.com/community/users/joon1/activity
size=+1[/size] https://www.sitepoint.com/community/t/pink-box-spreading-enough-having-breaks-in-text-in-yellow-box/212679