Ahh. This is why I like numbers better. Terminology varies from place to place, but 25g is 25g everywhere.
If you're not sure of your ability to eyeball food yet, you can weigh it. Weigh your snake (in grams), and weigh the food. If your snake is relatively young, a general guideline is 10% to 15% of bodyweight. Although that is by no means a hard and fast rule, it is a very good guideline to start with. Once you weigh your food a few times and get some experience with the numbers backing you up, you'll get more confident in your ability to eyeball the food.
If you want, post both weights here and someone will let you know if you are way too big or way too small.
P.S. The food in that pic is too small. She can definitely move up in size. Don't be fooled by the head and neck...if it fits in the middle of their body and it's rodent shaped, it will fit down the mouth and neck.
Who goes to pet shops and says "oh, let me break my scale out and weigh it first".