First of all equal height is an option to make the item container size equal. It's not for images to resize.

To achieve visually same size in carousel module, your images need to be meet one of the two criteria.

Option 1. Images need to be the same size in height and width. 

Just for an example 800 X 600 (All Images), 640 X 480 (All Images), Or whatever size you want.


Option 2. Images aspect ratio need to be the same.

What is aspect ratio?

An aspect ratio is a proportional relationship between an image's width and height. For example 3:2, 4:3, 1:1 etc all are the ratios. 

An image can be different in size for the same ratio. 

For example aspect ratio 4:3

Image size can be

1200 X 900

800 X 600

400 X 300 etc.

Here is a ratio calculator-

When you use the same ratio with different size of images Our carousel will show the same size visually.