Ohhkay. In that case, just draw it at whatever size you're comfortable with. Maybe go for the vertical scrolling format if you want, or just the print format that we see in comic books in manga. This could be 8.5 x 11, 8.5 x 14 . . . whatever works for you. When you make a new file, be sure to set the dpi to something between 300 - 600. Default for most programs is 72 dpi which is just so godawful lol. This is possible in SAI (which I know you're using lol) when you make a new document, the setting is right there.
You can't do image-resizing in SAI, HOWEVER, you can do it in Photoshop, and I believe also GIMP and Fire Alpaca. I'm not sure the process for Fire Alpaca, but for Photoshop, you just go up to the menu bar, hit "Image", then go down and select "Image Size" (NOT Canvas size.)
A pop-up window will appear and you can change the size settings. Make sure it's set to pixels and not inches or anything else, then change the width to 940 px. If the height is longer than 4000 px, then just lower the width a bit more until the length is as close to 4000 px without going over (so 3999, 3800, whatever the closest amount is when you change the width).
Photoshop will change the size, then you can save the JPG for Tapastic uploading (keep an original copy of the original size though! Don't save over the original PSD with the size change!! This isn't required but it's highly recommended as I said already, so you always have a high-quality, full size version of your art and panels.) Once the JPG is saved, you're good to upload!
If you're not using Photoshop, then check out those free programs like GIMP and Fire Alpaca. All these programs are built with similarities to Photoshop in mind, and should both have some kind of image resizer. And yes, these programs are actually, legitimately free (not torrenting free if you know what I'm saying cough cough, you should not be running into any purchasing fees or anything as far as I know.) The process to resizing images shouldn't be too different from Photoshop, but if you can't find it, a quick Google search should solve your problem (maybe do this before you try out GIMP or FA so you know beforehand lol)
Hope that helps, let me know if you have any other questions!