I tried to reproduce this problem it seems like the behaviour for seeing the </> icon changes for small images. Default behaviour for regular sized images is to see </> after mouse cursor hover and with really small images I was managed to get </> visible using a left click on the image where left click is placed on left top or bottom corner. Mouse cursor must remain in normal shape in order it to work. I tried this with Obsidian 1.13.7, Installer version: 1.9.14 and macOS. Result might be different for other installer and versions. If you are still having problems, consider posting in Bug reports.
If you’re on a device with a keyboard, use the arrow keys to select the image then press Enter to reveal the text. Or press Tab to place the cursor in position to edit the embed size.