Can you lookup the value in a key-value array with the value in an entry field?
Two options here ...

{% set transformParams = { width: 1250, height: 1250 * aspectRatioSetting[entry.aspectRatio] } %}


Or just give the select field a value that represents the number, not the word. So set a value of 0.75 instead of four_three, and let the label be Four/Three (or whatever seems fit).