These are small free embroidery fonts made to work for 4 mm or less, but some requirements are necessary.
Fonts here in DST EMB AND .ESA FILE

Here is my suggestion:
For Dress Shirt Material Style:

• center, 1mm length 
• 0.17 pull compensation
• shortening and fractional turn-off
• for 75 thread 0.40 density
• for 65 thread 0.45 density
• for Frosted Matt 0.50 density
• for 40 thread 0.55 density
• Needle 60 or 55 for best result

For Polo Material or Cotton

• double zig-zag underlay 1 mm spacing and 6mm length
• 0.17 pull compensation
• or 75 thread, 0.40 density
• for 65 thread 0.45 density
• for Frosted Matt 0.50 density
• for 40 thread 0.55 density
• Needle 60 or 55 for best result

The sample picture here folow the sequence 75/60/frosted/40 thread. 
And made with 2MM tall!

Another 3D Font for your collection, totally free and tested!