Frontpage thumbnail size

Hi!
Thanks to anyone that could give me some help.
I would like to increase the sizes of the thumbnails on my frontpage to about one and a half times more than now and have them in three columns (more or less as seen here https://www.thomaspetherick.com/

Could anyone give me informations as to how to do that? It is really only about sizing and spacing.

https://www.taramabiala.com here is what it looks like now!

Many many many thanks to anyone who could help!