You will likely render many images and different sizes depending on where you want your final image to be seen/used. While editing, you will want to use different sizes depending on the stage of editing you are at. This article gives guidance on how to use the various sizing mechanisms within Fractorium.
The first stage is editing in the main window. Because you are making many adjustments here, it’s better to keep the image smaller. A quick way to make the image match the available space is to double click on the width and height fields.
There is a defect in Qt which causes the interactive renderer to slow down dramatically when another part of the UI obscures the image. This happens when the image is larger than the available area and scroll bars appear. So try to have your image be less than or equal to the available area.
Regardless of the size you use, the width to height ratio should match what you want your final image to be. When you open the Final Render dialog, you’ll have an opportunity to scale the image by a certain percentage, or adjust the size by an absolute amount.
When editing keyframes for animations, you should use a size that matches the display you intend to show the animation on. Common dimensions are 1920 x 1080 and 3840 x 2160.