How can I make portrait photos have the same width as landscape photos on portfolio single pages?

You would need to customize it via a child theme. Please do:

1. Create a new child theme for this customization. For how-to instruction, please see: https://uxbarn.ticksy.com/article/8649/

2. In the parent theme, open the "/includes/plugin-codes/custom-uxbarn-portfolio.php" file and find this function:

function enso_print_portfolio_image_format_content( $image_id_list_string ) {
   ...
}

3. Copy and put it into the "functions.php" file of the child theme you created in step 1.

4. After that, find this code in the function:

$image_size = 'enso-port-single-portrait';

Change it to:

$image_size = 'enso-full-width';

5. That's it. The theme should now display portrait photos in full-width.