Hi,
I would strongly recommend that you do the embedded tutorials in the Fusion Documentation and also some of the
Self-Paced Learning to help you to learn fusion faster and better. They can be found here:

It is also much easier for the forum to help you if you attach your file AND a screenshot of what you want to achieve
and what the problem is. You can create a file to export like this:

Time spent on the tutorials and self paced learning will not be wasted. Also check out the three RULES that are pinned to
the forum for further guidance.
What do you mean by "simply size a face"? The example you have given has a circle of a defined
diameter that you have extruded. I know the diameter is of a defined length because the sketch
is fully constrained. You can edit the sketch at any time and change the "size of the face" simply
by clicking and editing the Diameter Dimension of the sketch.

So then we come to the thickness of the disk you extruded from the sketch. You
would have entered a value of click and dragged to create the initial disk.

To edit the thickness of the disk you would click on the extrusion on the timeline and
select edit feature and either click and drag or enter a new value.

Beyond this explanation, I have no other idea about what you mean by "simply size
the face".
It is very obvious that you need some help, hence my initial part of this post.
Start by reading the Fusion documentation that has the information you need to start
learning. This documentation also has a series of embedded tutorials so you can read
and then do. Once you have completed reading the documentation, start doing the Self
Paced Learning. Time spent on these specific activities is NEVER wasted.
Check out the Fusion RULES #0, 1 and 2. These are pinned to the top of the forum.
Cheers
Andrew