New Messages
File > Options > Mail
Under Compose messages, click Stationery and Fonts.
On the Personal Stationery tab, under New mail messages, click Font.
On the Font tab, Select font, style and size.

Replies and Forwards
File > Options > Mail
Under Compose messages, click Stationery and Fonts.
On the Personal Stationery tab, under Replying or forwarding messages, click Font.
On the Font tab, Select font, style and size.