Is there any way to change the font size in receive email in Apple Mail?
The easiest way to do this is to "zoom" the message. Pressing <command>-equal(=) will increase the size.
There's a "sticky" setting in Preferences > Fonts & Colors for mail font size. Note that this will affect how a message appears when a user is composing mail too, but the large font size will NOT be visible to the recipient. In other words, if I change Fonts and Colors > Message Font to size 16, both incoming and outgoing messages appear at size 16 on my monitor; but when I send the message, recipients will see my outgoing messages in the normal, plain-text font. (The only exception is signatures, which maintain the formatting as it appears in both preferences and in the mail message.)