You should specify what colour you require and link to your board so the designer has a good idea on what buttons to design so they compliment the rest of your design.
If by the offchance the buttons have transparent areas they will need to know what background matte colour to choose so the buttons look seamless on the background to which they will be applied onto.
Goodluck with your request but more information gives your request a stronger chance of being filled and not to mention designed correctly.
Edit: Looking at your diagram your looking for a button to be around
100px width by
45px height.