Both of these plain MDF placemats are 3mm thick - and
 
Round is 25cm diameter
 
Rectangular is 29cm x 21cm
 
These have far more uses than placemats - they also make excellent door plaques, stick fabric on them to make personalised mouse pads, paint with blackboard paint for a wee old-fashioned slate (and use either chalk pens, or chalk  - available with us).  Small personalised blackboards are great for use with markets, in games or hanging in the kitchen.
 
A really fun idea is to glue maps, or parts of maps onto the placemats and then cover with a clear varnish when the glue is dry.
 
A great item to have in the craft box to encourage inspiration!
 
We recommend that you prepare the surface of the placemat with white paint or gesso first, and allow to dry before painting with either acrylic paint (such as the Chroma paints) or perhaps Posca paint pens.  The white base-coat will ensure the colours are most vibrant!