This is the original Tiberius! Some info from its terrific maker, Starf (with minor edits for the showcase format):
Tiberius the Tunica is the base for everything, and also the simplest shape a garment can be: literally just a rectangle with holes (= parts you don’t sew together) for the head and both arms. Worn only as-is, it looks like the proverbial sack of potatoes, especially with my choice of fabric here. Take a belt to it, and it already gives you much better looks (belt like this definitely not historically accurate).
Your colour choice and fabric quality will greatly influence the look here.
The length I chose here is just above the knee, which is typical for the Roman army (trousers - or breeches - were at some point adopted from Gaul/Germanic tribes, after being frowned upon as being “not civilised”). Civilians (citizens) also wore longer lengths, that covered at least the knees. There are some text passages that suggest that Roman women wore generally longer tunicae (up to floor length), and that this was considered “too effeminate” for men of the time [Keith, A.M., Edmondson, J. C..; Roman Dress and the Fabrics of Roman Culture, Univ. of Toronto Press, 2008].
The width also varied a bit, but generally ranged from about elbow-to-elbow to shoulder-to-shoulder, although it is sometimes hard to tell (for me) from paintings/sculptures if there is a separate sleeve or not. Going from the dating of the pieces, I assume mostly no sleeves. I chose a width that is somewhere in the middle, because I wanted a more streamlined version for my fighting and generally active character, but in hindsight I would have made it slightly wider for more mobility. The wider the tunica, the stronger the illusion of sleeves.
Since this is literally a long cylinder, it will depend highly on the individual proportions what a good fit will be.