How can I enlarge the font size to 18
You can't change the font size on here. The only way is if you use a zoom feature on your device. Sorry. :(
If you have a touchpad or touchscreen, you can pinch it to zoom. Simply use two fingers and move them away from each other on the pad/screen. If not, you can use your browser's built-in zoom function or get add-ons that can adjust page font sizes.
If using Firefox, click on the 3 horizontal lines in the top-right corner. Then select the + and - buttons next to "zoom" to adjust page size. Note that this will not change the font size itself, just the page zoom.
If using Chrome, there's a similar button in the top-right corner that also shows zoom controls.
Either way, the font settings aren't gonna change, but should help.
I did the pulling and stretching. But not everything needed is visible so I have to move the page this way and that it would be easier just to have the font size like I used to have in my old kindle
Font sizing is determined from the device in question, not Duolingo. Your old Kindle must have had more optimal settings. Afraid I'm not overly familiar with Kindles though. I just have a Kindle Paperwhite, which most would say doesn't really count...