For a larger build volume, what lengths of 2020 aluminium do I need?
Long story short: ratios can be ignored. You only need to let your firmware know actual size, if it needs so. However, using golden ratio seems quite reasonable.

When talking about rigidity, keeping it will become a real issue at some point and may not be always effectively resolved by merely changing 2020 to 3030. Using 4020 or double 2020 could be better solution to compensate the most common forces during printing.