changing board size in circuit studio

A very simple way to achieve this...

• Change to the Outline layer (or any other spare mechanical layer of your choice)
• Draw (using the Line tool) a rectangle of 200 x 200mm using thin lines
• Select the rectangle
• Use Home | Board Shape > Define From Selected Objects

Being new to CS, I assume you need more instructions than in the above reply.  The easiest way to make a square board:

1-Click on the Home Tab.

2-Set your snap grid to a divisible number to what you are trying to create.  You are trying to create a board that is 200mm^2.  Set your snap grid to 10mm, 20mm, 50mm or 100mm, etc. 

3-Click on the Board Shape button.

4-Select 'Redefine Board Shape'.

5-Now click anywhere in your design space.  This is one corner of your board.  Move your mouse 200mm up and click.  Move it 200mm right and click.  Move it 200mm down and click.  Move to the original location and click.  Press Escape and there's your board.

     You can see why snap grid helps.

     It also helps to move your PCB away from the edges of your workspace.  Keep a good margin from the edges.

6-Now set your board origin to the bottom-left corner of your PCB.  Click on the Origin button.

7-Select 'Set'.

8-Click on the bottom-left corner of your PCB.

     Sometimes it's to your advantage to place your origin in other places such as a mounting hole or the center of the PCB.  You can redefine the origin at any time.

A second method to create your board shape is quite useful and I use it often.  This is where you define the board shape from selected objects.  It helps you make complex PCB shapes.

1-Select any layer.  Preferably one that is unused like a mechanical layer.  You can change the name of your layer in the View Configurations if you like.

2-Draw what you want your PCB to look like.  Use any kinds of lines, arcs, circles, whatever.  Don't make the lines too thin or they might be difficult to select.  Don't make them too thick or they might give you a false perspective of your PCB.

3-Press 'Shift+S' to hide all layers other than the one you are using.

4-Highlight all of your lines that you just drew by selecting them with a box.

5-Click on Board Shape.

6-Select 'Define From Selected Objects'.

7-Press 'Shift+S' to enable all layers.

     Be careful to connect all lines in your drawing.  Unconnected lines will not allow you to define a PCB shape.

     If you want to hide your mechanical layer, right-click on the mechanical layer tab and hide it.  You can bring it back at any time by right-clicking on the layer tab and showing the layer. 

I had the same question and I found this tutorial very helpful in answering this question. It is from Altium.

https://www.youtube.com/watch?v=iX5Ad9dUvjo

See also a post by Squintz on 20 May 2020, with an answer by Adam Webber.

The "Use another layer" ... "draw the shape you want" ...  "Define from selected objects" strategy works well, is easy to remember and produces few mysterious puzzles inviting, "Why did it do that?" questions. 

It is not well documented and I thought that the "Redefine board shape" strategy was the only strategy available, so in my early days, I felt obliged to use it and became bitterly frustrated. 

The "draw the shape you want" strategy works easily for any sensible shape and probably for quite a lot of crazy shapes, such as a Christmas tree.  Conversely, when using the "redefine shape" strategy I often settled for the shape Circuit Studio would let me define, even what wasn't the shape I wanted.