Resize image blocks smoothly - or Size spacer blocks in smaller increments

Sorry, reframing the question.  Because of the specific requirements of a client's gallery, I can't use the built-in galleries so I am simply using image blocks.

And again, because of specific requirements, I have to adjust the size of some of the images to appear larger or smaller - even when their dimensions are close enough that they naturally align perfectly.

I need to create a small visual difference in size, so I'm either sizing a spacer between two side by side images or sizing the image blocks horizontally. But this does not fine tune - the size jumps either too big or too small.

I know that SP uses a column layout and that's what the spacer and images block is snapping to. 

Ideally what I want to do is to resize image blocks while constraining the image.  I can resize the image block horizontally, but it jumps.  Sizing vertically is the minute function I want, but it does not resize the image container horizontally at the same time so I lose the outer edges of the  image and that is a no go.  

Is there any css code I can use to enable more minute adjustments to the image blocks OR spacers? 

Thank you!

Can you share link to page where you use image? We can check easier