When drawing from reference photos, my source photo is often smaller than my finished drawing. While drawing proportions of the face, if something seems off, I like to check proprtions. Here's a simple method.
I pick a feature or section that's easy to measure. First I measure my reference photo. I measure the height of the face from bottom of chin to hairline for instance. Then I take the exact same measurement on my drawing.
(source photo) x = 3.5"
(drawing) y = 5"
5/3.5 = 1.428571428571429
Round up and move decimal 2 places to the right. This gives you the percentage to blow up your reference photo on a copy machine to match the size of your drawing.
In the above example, I would enlarge my reference photo by 143% on a copy machine to match the size of my drawing.
I use a piece of plexiglass and a washable marker.
I trace the outline and main features from my enlarged photocopy. Then I lay the plexiglass over my drawing and see what might be off.
This makes it easy to compare the drawing proportions to the traced features on the plexiglass. It's a rough outline, but enough to make a quick comparison and identify what might be off.
This helps me make minor adjustments and check my proportions. Obviously I could make comparisons using Photoshop or photo editing software, but this works for me. I hope this tip helps you when drawing portrait proportions.
One final note about tracing onto plexiglass. If you happen to use a dry erase marker, you may find it difficult to clean the marker off of your plexiglass.
Some sources suggest using alcohol to clean dry erase marker from plexiglass. Others suggest quickly tracing over the lines again with same marker and wiping off while wet.
A Magic Eraser will take it off but it dulls and clouds up the surface of the plexiglass. I found it easiest to use a fine tip washable marker.