Learn Finnish: Mitä enemmän sinä viivyttelet, sitä enemmän sinä stressaat.

This is the standard Finnish correlative comparative structure. Mitä is the partitive form of mikä (what), and sitä is the partitive form of se (it). When placed at the start of two clauses with comparative words like enemmän, they create the meaning 'the [more]..., the [more]...'.

What is the dictionary form of the comparative enemmän?

The base word is paljon (a lot / much). Its comparative form is enemmän (more). The mitä... sitä... construction always requires a comparative adjective or adverb right after those words.

What is the dictionary form of the verb viivyttelet?

The dictionary form is viivytellä (to delay or to procrastinate), which is a Type 3 verb. The -ele- suffix in Finnish often gives a verb a continuous, lingering, or repeated feeling.

Is it grammatically required to keep the pronoun sinä in both halves of the sentence?

No, you can safely drop sinä and just write Mitä enemmän viivyttelet, sitä enemmän stressaat. Because the verbs end in -t, it is already perfectly clear who is doing the action. Keeping sinä just adds a bit of emphasis.

Does the word order change in the second clause after the comma?