2 And thou shalt make the horns of it upon the four corners thereof: his horns shall be of the same: and thou shalt overlay it with brass.
2 וְעָשִׂ֣יתָ קַרְנֹתָ֗יו עַ֚ל אַרְבַּ֣ע פִּנֹּתָ֔יו מִמֶּ֖נּוּ תִּהְיֶ֣יןָ קַרְנֹתָ֑יו וְצִפִּיתָ֥ אֹת֖וֹ נְחֹֽשֶׁת׃
Upon the corners of the Altar there were four(4) wooden horns...
Here is a closer view upon these four horns...
The expression "horns of him"("קרנתיו"), and what is more important, the root "קרנ" almost certainly implies that these "horns" were pointy and curved.
This root is also usually used to imply - the horn of the ram or some particular breed of mountain sheep. This is not coincidental, as the expression "horns of him" ("קרנתיו") first appears in Genesis 22:13 in the very story about the Sacrifice of Isaac! (stunning, isn't it?)
Here are several images of rams' horns for you to compare:
This is a photo by Martin Stoltze of a Värmland sheep
The Värmland sheep is one of several old swedish breeds, all belonging to the group of nordic short-tailed sheeps. Fleece color and horn shape vary. This is a young white ram with large and wide horns.
And this is a photo by Evelyn Simak of a Norfolk Horn ram at a sheep show.
As you can see, the shape and size of rams' horns can vary dramatically between the species, so at this point I'm just going to use an arbitrary shape to illustrate the horns of the Altar - like this:
The Altar wooden Frame and its wooden Horns were then overlayed with reddish copper...
Here is a closer view on the resulting overlayed frame of the Altar...
And here is a close-up of the corner of the overlayed frame with one of the overlayed horn...
Description does not state explicitly the position, dimensions and the shape of these horns. However, I more than sure that these horns were located close to the top corners of the frame of the Altar, or they were coming out of an actual top corners of the frame (as I show on my renderings).
As to the shape and dimensions of these horns: they were most likely no more than 0.5 cubit tall and 0.5-1 cubits wide. I'm not sure about their thickness, but I think it was most likely about a figerbreadth to an inch.
Their shape was either resembling a flat spiral or flat helix (so as not to disturb the symmetry) and these horns were most likely positioned at 45 degree angle relative to the corners of the Altar.
But yet again, I have to say that more research is needed to fully answer address these question.