I am not a research person. However, some practical thoughts tells that the size of the fibre should not influence the abrasiveness directly. It may be more influenced by the concentration or density of fibres, type of fillers and its size and the matrix material assuming that one uses same cutting tool and cutting parameters. If concentration is measured as gms/ cc while making, 8 micron fibres would be relatively less in quantity as compared to 5 micron fibre. This may help a bit.

Carbon/graphite is typically used as a lubricant, so the carbon itself should not be directly causing much wear on the tool. However, I would expect that the interface between the carbon and the polymer is where most of your tool wear happens. By that logic, smaller fibers at the same volume percent (or weight percent) loading should cause more tool wear than larger fibers. It's difficult to say if this will be true in your specific case, because (as Ajay mentioned) there are many variables.

In general, tool wear is greatly influenced by the type and volume loading fraction of the fibers. Now, with same type and volume loading fraction, carbon fibers normally break in a brittle manner under critical bending stresses and produces abrasive debris that could be smaller in size than the fiber diameter. This means that the machining process of the two sizes of carbon fiber as you mentioned in your question produce debris with particles size distribution (PSD) of less than 8 micron and 5 micron. In this case the value of the average debris size at 50% (D50) will be probably little higher in case of debris size 8 micron and so will cause accelerated wear by abrasion of the tool edge at relatively little higher rate than the case of 5 micron fiber diameter.

Best regards