Learn Spanish (Latin America): Es muy manejable para ser tan diminuto.

We use para here to set a standard of comparison or expectation. By saying para ser tan diminuto, you mean 'considering it is so tiny, it is surprisingly manageable.' If you used por, it would imply a cause, meaning it is manageable because it is tiny, which changes the meaning of the sentence completely.

Why does the Spanish sentence use the infinitive ser instead of the gerund siendo for the English word 'being'?

In Spanish, whenever a verb immediately follows a preposition like para, it must always be in the infinitive form. So, while English says 'for being,' Spanish requires the infinitive para ser. You can never say para siendo.

Could I say muy diminuto instead of tan diminuto?

It is much more natural to use tan (so) here. The word diminuto already means 'extremely small,' so adding muy (very) can sound a bit redundant, much like saying 'very tiny' in English. Tan works perfectly because it emphasizes the extreme size as the standard for your surprise.

Can manejable be used for abstract things like situations, or just physical objects?