In general, I offer 3 sizes (width) for the big designs: 20, 25 and 30 cm.
The ideal size to center on the front of a T-shirt is the distance between your nipples. If the design is much present on the sides (for example, a rectangle), try not to add more than 2 cm to the measured distance. If it is barely present on the sides (like some lines or small designs that will not change the overall design if they do not show), you can go up to + 4.
To find the measurement easily, take a sheet of paper A4 and place it on your chest:
one side is 21 cm and the other is 30 cm. Turn it till you know what is the most adapted dimension.
Pocket (on the heart): 10 cm
Sleeve: 5 cm
Back of the neck: 8 cm
This is for the front of the T-shirt / bodysuit.
Newborn: 8 cm
Baby: 12 cm
Child: 16 cm