Sorry for the delay, but only came across this enquiry after looking at the new website; anyway to try to answer your question, it is like every ergonomics answer – it depends! It depends on what you're doing, who you are, how repetitive and how long you use it for. However, as a guide Mital and Kilbom (1992) suggest a max weight of 1.75kgs for precision hand tools.
– Mital, A., & Kilbom, A. (1992) Design, selection and use of hand tools to alleviate trauma of the upper extremities: Part I – Guidelines for the practitioner. Int. J. of Ind. Erg. 10, pp 1-5.