CST - How can I reduce the number of mesh cells in simulation?

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Hi! I am relatively new to CST Studio Suite. I am using the Student Edition, which hugely limits the number of mesh cells allowed for a simulation. In time domain calculations, for example, it allows only 30,000 mesh cells. Unfortunately, this is a ridiculously low number of allowed mesh cells and barely any of my simulations can run due to this limit.
My two current methods I've been using thus far are:

1. Using the symmetry options in the Simulation -> Boundaries menu.
2. Changing the Global Mesh properties in Simulation -> Mesh section of the ribbon to roughly 1 cell per wavelength.

However, these methods don't always work. For example, my currently set-up simulation is a helix, which takes up a very small space yet has over 30 million mesh cells for simulation.
My working plane and bounding area is for some reason huge. Is there a way I can reduce this? I currently use the "Open" boundary option, as opposed to the default "Open (add space)" option.
Is there another way to reduce the number of mesh cells in a calculation? At this stage I'm not too worried about resolution of the simulation; I just need it to run!
Thanks in advance!

Hi there,
You can further reduce the bounding box size by setting a fixed value for your box. This will of course affect your resolution by reducing it from the default lambda/4. In the open boundaries setting, instead of using "fraction of a wavelength" (default 4), use a fixed value.
Hope that helps!