Not too much is written on this matter, because the Maiar, being divine spirits, are not necessarily bound to a single body, and thus could, theoretically, be clothed in bodies that are powerful physically one day, and others the next. However, the Maiar are usually aligned with some sort of power or aspect, and the forms they choose reflect that. Some aren't more powerful than humans, some are much more impressive.
Here are some Maia whose physical powers we are familiar with.
Gandalf, who was known as Olorin in Valinor, is a Maia of Lorien, who learned kindness and patience from Niena. Of Gandalf, it is said that he took the form of Man and Elf when walking among the people of Middle Earth, and thus we can assume that his physical power is at least equivalent to those. Despite seeming aged, we know that he is physically fit as a much younger man, or even superhumanly so, in a pinch: we see him riding for days in The Lord of the Rings, wielding his sword, standing tall in front of Theoden, facing the Balrog and carrying Faramir, a not-small man, with ease:
Then Gandalf revealed the strength that lay hid in him; even as the light of his power was hidden under his grey mantle. He leaped up on to the faggots, and raising the sick man lightly he sprang down again, and bore him towards the door.
(That last from The Return of the King, ch.7, The Pyre of Denethor, and thanks to @JimmyShelter for the reference)
The Balrog of Moria, Durin's Bane, and the rest of his Balrog companions were also Maiar, described in the Valaquenta, the second part of the Silmarillion, as:
[T]he Valaraukar, the scourges of fire that in Middle-earth were called the Balrogs, demons of terror.
With their whips of flame they smote asunder the webs of Ungoliant, and she quailed, and turned to flight, belching black vapours to cover.
Nothing explicit is mentioned about their physical power, either there or in the Lord of the Rings, but they are described as beings of might, inspiring awe and fear in allies and enemies alike. In almost all cases where someone encounters a Balrog - including some of the mightiest Noldor of the First Age, like Feanor or Maedhros, they are defeated by the Balrogs.
Eönwë doesn't get a lot of screen time, but he does lead the forces of Valinor against Melkor and his armies during the War of Wrath, at the end of the First Age. He is described in the Valaquenta as:
Eönwë, the banner-bearer and herald of Manwë, whose might in arms is surpassed by none in Arda.
Implying that he is not powerful in a mountain-moving way (the way that Tulkas, of the Valar, is), but he could outfight any opponent, mortal or Maia.
And finally Sauron, everyone's favorite Valar wannabe. Sauron was originally a follower of Aulë, as can be seen from his skill at crafting rings later on, but he was also quite impressive in battle, as can be seen during the War of the Last Alliance at the end of the Second Age. In the LotR movies he is portrayed as standing 20' tall and cutting huge swathes through the ranks of elves and humans, but in the text, specifically of the Akallabeth, this is slightly more toned down:
But at the last the siege was so strait that Sauron himself came forth; and he wrestled with Gil-galad and Elendil, and they both were slain, and the sword of Elendil broke under him as he fell. But Sauron also was thrown down.
So Sauron was certainly more powerful than the average man, and could out-wrestle (I'm assuming we're not talking Greco-Roman here) the High King of the Elves and the scion of the Kings of Numenor, but he was cast down. And after the ring was cut off, he lost even that impressive body, and could afterwards not muster even that amount of physical power.
All in all, the Maiar's physical power varied. Melian or Saruman could probably not face a determined warrior in single combat, but others could be mightier than most mortals. This wasn't necessarily indicative of their overall might, but of their personal roles and orientations.
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