Learn Farsi: باید ابعاد یکی را با دیگری مقایسه کنید تا مدل مناسب را پیدا کنید. - You must compare the dimensions of one with the other until you find the right model.

They are a set pair meaning "one ... the other". yeki refers to "one of the items" and digari means "the other one". You use them together to contrast two specific things without having to repeat their names.

Why is the direct object marker ra placed after yeki instead of ab'aad (dimensions)?

The verb "compare" affects the whole phrase "the dimensions of one" (ab'aad-e yeki). In Farsi, the marker ra always goes at the very end of the complete noun phrase, so it attaches to the last word of that phrase, which is yeki.

Why are both verbs in this sentence (moghayeseh konid and peyda konid) in the subjunctive form?

The first verb takes the subjunctive because it is required by the modal word bayad (must). The second verb is subjunctive because it follows ta, which introduces a goal, purpose, or intended outcome that hasn't happened yet.

The English translation says "until you find", but can ta also mean "so that"?

Yes, ta can mean both "until" and "so that / in order to". In Farsi, both meanings use the exact same structure (ta plus a subjunctive verb) because they both express an intended future outcome. Either English translation fits perfectly here.