First of all, welcome to Tapastic! 
The 940x4000 is just the maximum, you don't have to fill it out. I suggest, for the best quality, to max out the 940 width tho to get as much data as possible
As for height, I personally use a standard of the proportions of A5? Like, work with an A5 canvas and post process it to fit the 940 width. To be quite honest, any would do as long as it works out for you 
Anyways, here's some updated tips I've been giving new comers ^^
-During the weekend, the days with the most traffic, post around early morning (9-10AM PST) to early evening (5-6PM PST)
-During the weekdays, the days with the relative traffic, post around early evening (5-6PM PST) as that's when people check Tapastic, after work/school'
-Note that this is mostly affecting the Fresh Page, which not many people go through regularly thus...
-Post consistently, have a schedule and post once or twice a week according to the above, I recommend twice a week to get a better weekly traffic flow
-Don't expect your views to relate to your subs, your subs will grow over time much like your views, but it'll be slower
-It takes around 3 years to get a stable series flowing here, but don't be disheartened and keep at it as things will progress faster than you'll expect
-Disregard the last point if you're adding "BL" on your icon (which is Boy's Love, a very popular genre here, for which this part is a joke unless you plan on making that kind of content ^^")
-This is very important: Keep posting on the forums and help people grow as they'll also help you grow both in viewership as well as an artist. It also allows you to reach more potential readers/subscribers as I found that more people check the forums for new comics rather than the Fresh page, where you'll most likely end up in on the main site
-Lastly, the algorithm is still something I'm studying, but so far, if a lot of people comment, like, sub and view your comic, there's a high chance of seeing yourself on the Trending section of the main page, even just for a brief refresh
Anyways, if you like Mysteries, check out my comic, Jim's Days ^^