What is the smallest molecule that can present as an antigen to the immune system in the context of allergies?
At one of my previous companies that raised antibodies to proteins and post-translational modifications, we found that 6-8 amino acids was generally the smallest peptide length required for an epitope. Anything much smaller than that won't induce the cross-linking or conformational changes required for signal transduction and an allergic response. As WYSIWYG notes, allergies to small molecules/atoms are formed via hapten-carrier interactions. These can be covalent, as in the nickel-transferrin example, or as a result of denaturation/chemical modification, as likely happens with people who are allergic to things like isopropanol or bleach.

I very much doubt that there can be such a thing as a glucose allergy, as it is so ubiquitous in the body that such antibodies/T-cells would be negatively selected long before they could do any damage.