Though there are standard pre-defined material sizes in the Silhouette program (such as 8.5 x 11 Letter size and A4 size), you can define custom user paper sizes with the Silhouette Studio® software to fit the media type and size you're working with providing the material size is allowed by the scope of what the Silhouette can handle.
The Silhouette Cameo® has adjustable pinch rollers to control the material width allowance with pre-set roller settings for A4 width, Letter width, 12 inch material width, and 13 inch width (in order to grip the 12x12 inch cutting mat). It cannot accept any materials with a width exceeding a 13 inch path.
The Silhouette Portrait® has pinch rollers set to accept A4 and Letter size materials, as well as the standard Letter size cutting mat (which may be used for A4 size materials as well). The pinch rollers on this model are not adjustable. It cannot accept any materials with a width exceeding a 9 inch path.
When cutting longer materials that have their own adhesive backing (such as adhesive-backed vinyl), you may cut lengths of up to 10 feet so long as the material is wide enough to be gripped by the pinch rollers.
Materials that are smaller than the width of the pinch rollers would require the use of a cutting mat to hold the material in place properly during the cutting process. If such is the case, the maximum length of the material could not exceed that of the cutting mat (12 inch maximum length for the Cameo when using the 12x12 mat or 24 inch maximum length when using the 12x24 mat; 12 inch maximum length for the Portrait).
The smallest recommended size of material would be at least 3 inches width by 3 inches length (which would require the use of a mat as they are not wide enough to be gripped by the pinch rollers).