I usually write poorly, but now I practice hard pen calligraphy. Why do my characters have different sizes in Chinese characters? Is there any way to write every word?

Take your time, calligraphy needs precipitation and accumulation. Write a few words every day and try to start with strokes. The other is to learn to feel the inner charm of calligraphy, follow your heart, write naturally and affirm yourself! Hehe, when I was a freshman, my handwriting was like that of a primary school student. Much better now, mainly because I find the charm of calligraphy attractive, hehe. I suggest you practice block letters more first.