Learn Icelandic: Trén eru mjög stór.

tré is a neuter noun. Neuter nouns often look exactly the same in the plural, so tré can mean "a tree" or "trees". To say "the trees", we add the neuter plural definite ending -in. Because tré already ends in a vowel, the i in -in drops out, leaving us with trén.

Why is the adjective stór used here? Isn't that the singular form?

It looks like it, but stór is actually the neuter plural form here! Adjectives must match the noun they describe. Because trén is neuter and plural, the adjective takes the neuter plural form. For most Icelandic adjectives, the neuter plural happens to look identical to the base dictionary word.

Why do we use eru instead of er in this sentence?

eru is the plural form of the verb "to be" (are), used because we are talking about multiple trees. You would use er (is) if the sentence was about just one tree: Tréð er mjög stórt (The tree is very big).

Do the accent marks on trén and stór mean we put more stress on those letters?

No, in Icelandic, an accent mark changes the actual vowel sound, not the stress. The é in trén sounds like "ye" (as in "yet"), and the ó in stór makes an "oh" sound (like in "slow").