Learn French: Ce diamètre, je le trouve parfait. - This diameter, I find it perfect.

Because this sentence isn't stating a specific numerical measurement. If it were, you would use the lesson's structure: un diamètre de 5 centimètres. Here, the speaker is just referring to the diameter as a concept or feature they are looking at, so no de is needed.

Why put Ce diamètre at the beginning and then use le? Couldn't we just say Je trouve ce diamètre parfait?

You absolutely can say Je trouve ce diamètre parfait. However, moving the topic to the front (Ce diamètre) and echoing it with a pronoun (le) in the main clause is a very common spoken French habit called "dislocation." It emphasizes what you are talking about before you deliver your opinion on it.

Does parfait agree with the subject je or the object le?

It agrees with the direct object le, which represents the masculine singular noun diamètre. When using trouver to express an opinion about something, the adjective always agrees with the thing you are judging, not the person speaking.

Since we use ce and le, diamètre is masculine. Are most nouns for physical dimensions masculine?