Hi I have done ELISAS with peptides of 10 amino acids and it works just fine, no problem at all.

ELISA shows no limitation for number amino acids but it is widely concerned with minimum quanity of a protein or a peptide i.e should be in a range of 0.0001-0.0010 mg/mL. It can also determines single amino acid peptide of 5 monomers in one. 

Dear Habiba Soliman,  no length limit for ELISAS, Ravi Kant Upadhyay sir gave the clear answer for your question..

ELISA will work and give perfect information even for a dipeptide to any big protein in that no doubt, but you should need to give lesser concentration range (0.0002-0.0010mg/mL.

Hello,

To complete these answers, I would add that it also depends on the ELISA format: actually, two main formats do coexist, which are the sandwich format (or immunometric) and the competitive format. For small molecules such as peptide, the suitable format is the competitive one since it involves only one specific epitope regonized on the molecule to assay. And indeed, there are some commercially available kit able to measure 9 amino-acids long peptides, such as oxytocin or vasopressin.

Best wishes