3" diameter Mini marker with a bottle opener and magnet.
These are legal size for use in PDGA tournaments as a marker to mark your lie. From the PDGA technical standards:
These are legal to be used as a mini marker in PDGA play, but not as a mini for mini golf competition. From the PDGA:
"Mini Marker discs must have a circular shape, with a diameter ranging from 7 cm to 15 cm and a height not exceeding 3 cm. Mini marker discs can be made from a variety of materials (e.g. plastic, metal, wood). The Mini Disc Golf Federation officially defines mini discs suitable for mini golf competition (http://minidiscgolf.com/rules/). "