Hi Cloel
The best way to do this, is Select your ‘Scale Box’ and in the Details Panel, make sure the ‘Stretch’ option is set to ‘Scale To Fit’ and ensure the option below that ‘Stretch Direction’ is set to ‘Down Only’.
This way, the text will only scale down and not up. So it will remain at the size you specify until it fits the whole Box and then will scale Down to fit more. So even if you made the size box bigger, it won’t scale up, it will keep it’s size.
You can then use the Vertical Alignment of the ‘Text’ up the top in Details to be set to Middle or Top etc depending on where you want your text displayed inside the scale box.
Hope this helps.