About twice the height of an Elf
Depending on you views of the canonicity of the Book of Lost Tales - they are basically the draft notes for the Silmarillion.
(personally if they are not contradicted by later canon I tend to take them at face value)
Assuming that Balrogs are the same size the Balrogs at the Fall of Gondolin appeared to be of a consistent double elf height.
Then sprang the Balrog in the torment of his pain and fear full at <PERSON>, who stabbed like a dart of a snake; but he found only a shoulder, and was grappled, and they swayed to a fall upon the crag-top. Then <PERSON>'s left hand sought a dirk, and this he thrust up that it pierced the <PERSON>'s belly nigh his own face (for that demon was double his stature)**
***- The Book of Lost Tales - Part II - Gondolin*** Also from the Book of Lost Tales when talking about the Fall of Gondolin > *Then leapt <PERSON> lord of the Fountain, fairest of the Noldoli, full at Gothmog even as he raised his whip, and his helm that had a spike upon it he drave into that evil breast, and he twined his legs about his foeman's thighs; and the <PERSON> yelled and fell forward; but those two dropped into the basin of the king's fountain which was very deep. There found that creature his bane; and <PERSON> sank steel-laden into the depths, and so perished the lord of the Fountain after fiery battle in cool waters.*
***- The Book of Lost Tales - Part II - <PERSON>*** <PERSON> was obviously of a height that a severely injured <PERSON> was able to drive his spiked helm into his chest. Too much bigger than the elf and this wouldn't have been possible. <PERSON> was also *Lord of <PERSON>* so it could be expected and assumed that he would be one of, if not the, biggest of the <PERSON>. If the <PERSON> were too big one would not expect Elves, nor a man sized <PERSON> (though <PERSON> powers could be at play here), to be able to go toe to toe with them on numerous occasions For example <PERSON> was surrounded by <PERSON> until he was eventually worn down > *and there issued from <PERSON> to aid them. There upon the confines of Dor Daedeloth, the land of Morgoth, <PERSON> was surrounded, with few friends about him. 88
Long he fought on, and undismayed, though he was wrapped in fire and wounded with many wounds; but at the last he was smitten to the ground by <PERSON>, Lord of <PERSON>, whom <PERSON> after slew in Gondolin.*
***- The Silmarillion - Of the Return of the Noldor*** The <PERSON> were then driven off by <PERSON>'s sons. <PERSON> was holding his own against <PERSON> until he was assaulted from behind: > *At last <PERSON> stood alone with his guard dead about him; and he fought with <PERSON>, until another Balrog came behind and cast a thong of fire about him. Then <PERSON> hewed him with his black axe, and a white flame sprang up from the helm of <PERSON> as it was cloven.*
***- The Silmarillion - Of the Fifth Battle: Nirnaeth Arnoediad*** Then you have <PERSON> and <PERSON> killing a <PERSON> each in the defence of <PERSON> at the cost of both of their lives. Admittedly Elves in those days were greater and capable of great feats, look at <PERSON> fighting <PERSON> and wounding him several times, but there must still be a close enough difference in size for the combat to be realistic otherwise the elves would have been overwhelmed by sheer brute force. ---------- Possibly of interest -------------------- User <PERSON>*** over at [The Lord of The Rings Fanatics Plaza][1] has *worked out* the <PERSON> size based on the Bridge of Khazad-Dum. I will try to summarise his calculations here in case of link death, however I am in no way trying to take ***ANY*** credit for this. > <PERSON> stood in the middle of the 50 foot bridge, therefore he is 25 foot from the end of the bridge. The <PERSON> stops before stepping onto the bridge, putting it 25 foot from <PERSON>. 381