On the menu bar > Format. The Text Tool Bar should appear under the traffic lights in the centre of your screen. You can then select the font etc. and enlarge the text to any size you want.
Though you may find the receiving machine has default settings which may overrule anything you set. Or it may be that it's better to set your Composing (in Mail Preferences) to Plain Text rather than Rich Text to enable his machine to adjust it to his liking. Experimentation needed.
I know you have a solution now but you could consider typing the email into Pages choosing a font and layout that you like.. Then save the document as a PDF file and then email the pdf file to your contact.