How to use thumbnail size of image if I’m only using src to get image

Since you are using get_sub_field() i assume you are inside a have_rows() (a repeater), in theory it should work like this:

$image = get_sub_field('image');
<img src="https://wordpress.stackexchange.com/questions/278286/<?php echo $image["url']; ?>" alt="https://wordpress.stackexchange.com/questions/278286/<?php echo $image["alt'] ?>" />

and this is what you can get:

$image['url'] //this is the full image
echo $image['sizes']['medium']; //medium image
echo $image['sizes']['thumbnail']; //thumbnail image
echo $image['sizes']['yourcustomsize']; //custom sized image

another way is to use:

$size="full"; // (thumbnail, medium, large, full or custom size)
echo wp_get_attachment_image( $imageID, $size );

now if these dont work for you, that means your images are still saved as just the URL, not the object. try re-saving the post/page where the image is uploaded or if you have a set of images, you might need to use another plugin to reupload and create the sizes.