This is an all-over print shirt, with great Green Hill theme. The pattern takes the entire screen for the shirt's design, printing on every part front & back. Because it's kept so big, it's not overly pixelated, and the design appears clear, bright & colorful. Sonic's sprite is correctly sized for the background as well, further contributing to the good look. Sonic is in his 'foot tap' frame, near a red spring.
The good news is that this is a men's-size shirt, so it's range of sizes should fit anyone. (Just use a small for kids & teens) This is likely to be a Primark item. (unconfirmed)