Your mention of pencil made me think of pencil and paper, but it sounds like you’re using a digital pencil or pencil effect of some kind.
Large format printing is typically done at lower resolutions than 300 ppi because it’s usually viewed from further away than the 300 ppi rule of thumb used for printing meant to be viewed from arm’s length. It’s not all that often that an image with a higher pixel count than what you’ve mentioned is needed — even for something as large as a billboard.
I think your concerns are mostly unwarranted. Assuming you’re in Photoshop or a similar application, just display the image at various sizes to see what it looks like. If you want to zoom in on something, that’s fine too. Your linework won’t change in terms of how it looks; it will just get larger or smaller depending on how it’s scaled.
I’m not sure what you’re referring to when you mention “lines,” but if the lines are thin at a high resolution, keep in mind that they’ll be even thinner at a lower resolution. For example, if you draw your illustration at 8000 x 3000 pixels and use thin lines, those lines will be much smaller and thinner when scaled down to 1000 x 375 pixels. This isn’t necessarily a problem, but if the illustration depends on those thin lines being visible and keeping their integrity at small sizes, you might consider making them a bit thicker.
There’s no good rule of thumb for this. It’s a matter of common sense and trusting what you see on your monitor when viewed at different sizes. If it looks good to you when viewed small on your monitor and good when you blow it up to 100%, it should be fine.