There's a couple ways to go about changing the height.
Option 1 - Adjust height of images
You could use the built-in image editor (via the file manager) and you could crop or resize the images to a certain height. Or, just make sure all images are the same height when you upload them.
Option 2 - Edit source code
You'll need to edit the source code for the core Image Slider block.
Go to:
/concrete/blocks/image_slider/view.css
From there, you can edit the CSS all you need. You could do something like this:
Option 1 - Adjust height of images
You could use the built-in image editor (via the file manager) and you could crop or resize the images to a certain height. Or, just make sure all images are the same height when you upload them.
Option 2 - Edit source code
You'll need to edit the source code for the core Image Slider block.
Go to:
/concrete/blocks/image_slider/view.css
From there, you can edit the CSS all you need. You could do something like this:
Hope that helps.