Hi @nicolas, in the new Constraints section you can still use proportional values, they’re referred to as relative now, just click the little % percentage sign you see next to the constraint values.
If you hover over a constraint option you’ll see that each one allows you to specify size and position using a combination of the available values.
To make your element fill the available space you can use the center option you see in both horizontal and vertical sections, setting left, right, top, and bottom to zero. Alternatively you can use a relative width value of 100% and zero on either the left or right (for horizontal - for vertical you’d set a 100% height and zero top or bottom).
More on the constraint options in our help center: