TWSBI Diamond 580 Red Fountain Pen Nib F
A quick no brainer guide in choosing nib sizes in an overly complicated fountain pen world:
If you write small, go with an EF

For the normal writer, F is the most popular

If you tend to lean on a bigger font, M will be perfect

B is for statements!! Sign a check in B, write a header in B, do some coloring/shading in B

Stub nibs are italic in writing style (have a special card or invitation to write?)