pVIII is the main coat protein of Ff phages. Peptides are usually fused to the N-terminus of pVIII. Usually peptides that can be fused to pVIII are 6-8 amino acids long. The size restriction seems to have less to do with structural impediment caused by the added section and more to do with the size exclusion caused by pIV during coat protein export. Since there are around 2700 copies of the protein on a typical phages, it is more likely that the protein of interest will be expressed polyvalently even if a phagemid is used. This makes the use of this protein unfavorable for the discovery of high affinity binding partners.
To overcome the size problem of pVIII, artificial coat proteins have been designed. An example is Weiss and Sidhus inverted artificial coat protein (ACP) which allows the display of large proteins at the C-terminus. The ACPs could display a protein of 20kDa, however, only at low levels (mostly only monovalently).