Hello SigridMN and welcome to Apple Support Communities.
I see you are having some difficulty with the size of the thumbnails for photos when adding pictures to a Facebook post using a web browser on a Mac.
At this time, there is no option for increasing the size of the thumbnails. As a workaround, you can open the Photos app independently of the browser and choose the photo you wish to share. (In the Photos interface, you can increase the size of thumbnails using the slider in the upper left.) Double click the photo and then use Edit > Copy or Command + C. Then go back to the browser and your post and use Edit > Paste or Command + V to paste the photo into the post. You can also drag and drop the image from the main Photos interface directly on to your post.
Cheers.