When you add images using the Insert method, the size will depend on your choice in the Style dropdown selector in the File Information panel of the Add Images section. Various size options are available and you can experiment with them to get the best effect on your final page.
However, there is another trick you can use, especially if you have uploaded a large image.
On the Style dropdown, choose Original Image and insert it into the Body panel of the editing page as per the usual instructions.
When the image appears in the Body panel of the editing page, click on it and then right click. This will open a small dialogue panel with Image Properties at the bottom. Click on Image Properties and find the Width box and the Height box in the dialogue panel.
In the Width box, delete all numbers and replace them with 100% (as this is written). Delete any numbers in the Height box and leave it empty. Click on the OK button at the bottom of the Image Properties dialogue panel to close it.
Complete your post or comment as normal and don't forget to click Save at the bottom of the Editing Window to publish it.
Your images will now expand automatically to fill the maximum available space on the page but without spreading out over the rest of the screen. This is a good way to give other site users the best view of your images.