An error occurred while saving the commentBen Tupper commented
The plugin that will do this is called Trimmit. Free. Works.
I did a search inside XD's plugins panel (click the little + to open search) after selecting plugins (bottom left - looks like a lego piece) in XD's ui. I searched for "snap" (no quotes of course). It found something called Trimmit. I installed it to give it a try. It's free and installs in the XD plugins panel instantly. I went to the artboard in question which is 1280 x 800 (default Web 1280 XD artboard size). I clicked on it to select the artboard. I clicked the Trimmit button in the plugins panel. It instantly increased the length (from the top downward) to match the artwork (1280 x 3309) on the artboard.
Does exactly what I need as I take lengthy screenshots from a web app and import them into XD. I would imagine it Trimmit will make artboards snap to artwork for multiple artboards at a time, but I only tried it with one and it just worked.
Looks like this was created by someone from Adobe... here's the link to his GitHub page: https://adobe.com/go/xd_plugins_discover_plugin?pluginId=3a010606