The largest opening is about 6 1/4" x 4 1/2"". The largest of the smaller openings is 3" x 2 1/8". This quality matting material. Other size mats are available up to 16 x 20" Custom orders welcome. COLOR LIST: They are from left to right: Bright White, Light Grey, Medium Grey Grey, Soft White, Pink, Cream, Red, True Green, Spring Green, Forrest Green, Silver Blue,Royal Blue, Sky Blue, Navy, Brown, Tan, Charcoal, Gold, Silver, Black. Yellow not shown. RETURN POLICY All are mats are individually custom cut "one by one" for your order specifically. We do not stock them already cut . For that reason they cannot be returned but if you have a problem please contact us and we will see what we can do to resolve your problem. CHOOSING THE CORRECT MAT SIZE When purchasing a mat for a pi than the size of the mat you purchase. In other words an 11 x 14” frame should actually measure at least 11 1/8” x 14 1/8”. You mat will measure 11 x 14”. In addition there is a 1/16” allowance on the mat so it can measure 10 15/16” x 13 15/16” up to 11 1/16” up to 14 1/16” and still fit in your standard frame. Some less expensive frames may not have the allowance so you would need to ask for a size that is at least 1/8” smaller than the opening on the frame itself. All are mats are individually custom cut "one by one" for your order specifically. We do not stock them already cut . For that reason they cannot be returned but if you have a problem please contact us.