You have the potential to do all sorts of damage if you approach this without training. A key point is that in general you never change fonts or styles or similar in the obvious ways (from the menus/toolbars) but by changing styles. Change directly only if the original creator didn't use styles.
Another tip: sizes in InDesign are NOT in pixels, even if (very misleadingly) you can choose "Px" as a unit. Think of this as "nonsense pixels" as a unit. InDesign work is measured in inches or millimetres. If it is meant for web use, there are some tricks to saving with a predictable pixel count, though saving direct as PNG or JPEG from InDesign is problematic and may not be the best route.
Thanks for reaching out. As @Eugene Tyson suggested, click any page and check the properties panel on the right for the dimensions. Also, you can modify or check the dimensions by going to File menu > Document setup. I hope it helps!