It is correct that ISOM2017 specifies that dimensions for circles and lines increase proportionally with the map scale. This means that at 1:5000, circles should be two times the size of circles at 1:10000.
In Condes, the common way to handle multiple scales is to use multiple canvases; one for each scale.
For each individual canvas you can set circle and line dimensions via the menu Canvas / Circle and line dimensions. This lets you set the correct dimensions for the print scale of the canvas.
To simplify handling of multiple scales, there is a setting under Canvas / Controls to "link" dimensions between canvases. When you check the box "use the same dimensions", the dimensions on the canvas use the settings from the "main" canvas, and scale (enlarge/shrink) dimensions relatively to the print scales of the two canvases. This includes the dimensions of course symbols such as out-of-bounds, refreshments, etc.
So, on the main canvas you set the dimensions according to ISOM2017 (and save those to your standard settings to automatically use them for your next event. Then, for additional canvases, you check the "use same size" option to scale dimensions according to print scale.
Further support for ISOM2017 is planned for a later update.