Ok, here's an example how big the picture is. I have attached Sandman's portrait from WCS, I just converted it from PCX to PNG.
The original picture format is PCX, with a special green as a background for transparency, and it is pretty small: 160 pixels wide, and 120 pixels high.
What I would suggest is drawing it a bit bigger, but keep in mind that small details will vanish when it is shrinked to its target size.
But since I think that it would be a waste of talent to just show it that small I will also include the portrait in the story PDF, like Saga did in the manual (also attached).
And here's the description for my Kilrathi:
The style should be "semi-realistic", so not too exaggerated features or colors. I am aiming for somewhere between the look of the Kilrathi in WC3 and those of WC1. (more like WC3 though) In WC2 they look too much like real cats IMO.
I like this painting of yours, that's how a Kilrathi should look like IMO.
https://cdn.wcnews.com/newestshots/full/ninjalasketch14.jpg
The Kilrathi is called Ku'rash nar Kur'u'tak, a lesser noble. He should not wear a helmet (or maybe an open WC3 style one so we can see his face), it should be a portrait, perhaps just showing a little bit of his flight suit, a bit of his shoulders maybe. If you decide to draw him with a helmet, please draw an additional version with a dark closed visor, I could use that as well.
Or maybe the portrait is just part of a bigger picture so you would draw him standing (like Sandman in the WCS manual) and I would just cut the portrait from there for ingame use.
I think a 3/4 perspective will look better than a straight one, if you can get the perspective stuff right. I have no clue how hard that actually is, since I am not capable of drawing.
He is still a young Kilrathi, so he shouldn't have grey streaks in his fur or something. His fur should be not too short (perhaps not as long as WC3 Thrakhath's though), in a brownish color, and he should not look too aggressive (no bared fangs or something). If you like to experiment a bit you could try how he looks with some subtle tiger stripes in his fur (like WC3 Hobbes) or something like that. I'm not sure about the eye color, I'll leave that to you.
Here are some pics for reference:
https://www.wcnews.com/articles/wc3kilrathigallery.shtml
https://cdn.wcnews.com/newestshots/full/SMALLWC3SetPhotoMythicBox1Scan0261.jpg