Image size

Hi,
I don’t understand how works image size.
For my slider, I use the 3 images options, it works, but I have a problem with the image.
If I use the option 150×150, my 3 posts have the same size (but it’s not 150×150, it’as about my screen resolution/3) and the images are blurred.
If I choose 300×300, It’s better, the images are not blurred and smaller. But, the height of the image is different for each image (in 150×150, it was the same).

So what I have to do to have the same size for each image and images not blurred ? Do I have to resize manually each image with the same size ?

Thank you !