Latest Post and different custom image size

I’m currently building a website which needs to show the most recent post with a 50% width of the container, then the rest need to be at 25% width.

I am currently using css (first-of-type) to do this with a custom image size of 400px x 250px. But when the images are displayed they are getting being sized down and shorter than I would like them.

Is there a way to set 2 different image sizes and to call the first post with size1, then the rest size 2 (or in my example, track-first then track)?

https://pastebin.com/WLLBBvYq