For this, I recommend creating a Page (suggested units: 120 wide x 180 tall) that is your canvas, corkboard, matte, backdrop. Set a dark background color. This can be more of a storyboard / timeline / scrapbook method not limited to 1 scene per page. Place your theatrical scene image(s) around the Page, not entirely filling it edge to edge. Always leave a margin (recommended: 4 units) on the left, right, top, and bottom, which facilitates scrolling and zooming to all corners. In other words be sure to inset your images, not to fill the entire page with the picture. As GM on an average sized screen like a laptop you can experiment, go to 100% zoom (the default starting point) within the page and drag your scene image to fill the amount of screen that you like. A picture filling perhaps -- I'll get a unit number for you so I'm not guessing, brb -- filling something like 10x8 units should look like a nice centered picture on many screens at 100% zoom, and if it's not right-on then the person can zoom in or out a little bit. A picture around 20x12 could be an ideal compromise that would predictably fill most screens or at least be generously sharp at varying zoom levels. A picture filling something like 40x30 most people / most screens will be either zooming-out to see the entire picture, or scrolling around to see it all. To get into the gritty math part, as an aside, 1 unit in Roll20 at 100% zoom is 70x70 pixels. You can compare this with monitor / TV / and laptop screen pixel resolutions if needed to be precise. 1080p High Def screen is 1920 pixels wide, give about 30% of the width for chatroom UI, they would still have a tabletop view of around 1400 pixels, so they could see a picture 20 units wide (roughly 20x12) at 100% without zooming or scrolling. Note: Some laptop screens are smaller / worse resolution than this number, and some screens like the current 5K Macs or 4K TV's are much higher resolution. The GM can also now use Shift-Ping (shift and hold mouse button) to draw
everyone's attention to the center of the current scene image, just to
make sure they're on the right spot. It is up to each player to properly
refine their Zoom from that point, but given the margins we're giving
in this method, it should be possible to size it ideally on any screen
size. If you like, now turn on Fog of War, and use Reveal tool to show just the scene picture that you want everyone to focus on. This gives you (GM) some hidden side-border spaces to collect monsters or other surprises, or to prepare a full Storyboard of your Scenery images that would be revealed one by one, chapter by chapter --- by revealing on the Fog of War on the singular Page, and you won't necessarily need to make a new Page for each new Scene image. And this cues the player where to scroll and zoom, if the default didn't start out as much on-point as we're hoping. Bonus points, place additional scenery images (perhaps smaller sized) around the edges of your main/central image. GM can select the side-images and press Shift-Z to pop up the bonus picture for display in the middle of the screen at full size with a black lightbox effect. These could be portraits of people who enter the scene, or could be a note on the table and if they look closer at the paper and read it then you can show the adjacent image as a pop-up or by revealing it from the Fog. ------ The alternative, if you decide to make Pages where the Scene image fills the entire page, the good news is that Roll20 will automatically cause everyone's window to look at the correct spot, and there won't be any room for scrolling. (Also no room for the fog of war and side-images). In this case I would try something like 20x12 for the page units, and experiment from there. 10x8, to 20x16, something around that range should create a fully-viewable entire page.