We are told by Thucydides that at the beginning of the war Pericles encouraged the Athenians by giving them a full assessment of their financial and military power. He says in Book 2.13 that the Athenians had:

• 6,000 talents of silver in the treasury, + 600 per year collected in tribute from the Delian League. They also had c. 1000 talents in private hands and adorning statues and religious icons that could be melted in a crisis.

• 29,000 hoplites: 13,000 in Athens and 16,000 in garrisons across the Aegean.

• 300 battle ready triremes - so at the beginning of the war Athens alone could muster a fleet of 300 warships, that is without her allies (some of whom provided ships but by 431 most just contributed money to the league).

In 435 BC Athens also entered into a defensive alliance with Corcyra. When discussing the possibility of alliance Thucydides tells us that the Corcyraeans said they have the second most powerful navy in Greece, second only to Athens (Thucydides, 1.33). At the battle of Leucimme fought between Corcyra and Corinth in 435 the Corcyraeans had 80 ships to fight the Corinthian fleet of 75, whilst 40 more ships were engaged in the siege of Epidamnus nearby. That gives Corcyra a combined naval strength of 120 ships. In 433 BC at the battle of Sybota between Corcyra and Corinth again. The Corcyraeans fielded 110 ships.

So combined with the Corcyraean fleet after 433 BC the Athenian fleet could boast of around 420 warships, which would likely be the height of their naval power. This does not take into account the ships provided by other allies, which would not have been small in number when combined, so the number would be significantly higher than 420 in real terms were the entire combined Delian fleet to unite.

In 415 BC the Athenians sent out a fleet of 134 Triremes on the Sicilian campaign. Thucydides says this was the “most costly and splendid Hellenic force that had ever been assembled” (6.30). He compares it to the force Pericles sent out against Epidaurus at the beginning of the war: which consisted of “four thousand Athenian heavy infantry, three hundred horse, and one hundred galleys accompanied by fifty Lesbian and Chian vessels and many allies besides. But these were sent upon a short voyage and with a scanty equipment.”

By comparison in 415 BC the Athenians sent out a force to Sicily of 134 triremes and 5,100 hoplites plus allies. They were then reinforced in 413 BC with an extra 73 triremes and 5,000 hoplites. So a combined force of 207 triremes and just over 10,000 hoplites. As Thucydides said (above) this was the largest force they ever assembled. In 413 BC almost the entire expeditionary force was wiped out, this would be roughly 1/2 of their hoplites forces from the beginning of the war (taking into account the plague in the early 420s and defeats such as Amphipolis in 422 BC, but if we take into account the number of triremes counted above it would only be about 2/5-1/2 of their naval strength. Hence why they could endure so much longer after the defeat in Sicily.

Hope this helps!

Edit: some typos