When titrating to find the concentration of a dissolved acid, does the volume of solvent matter?
At very great dilutions, impurities could have some effect on the end point. For example, even previously-distilled water will absorb some small amount of $\ce{CO2}$ from air, making it slightly more acid. However, if the aspirin is present in reasonable amount, that would have negligible effect. So, as stated by @MaxW, consistency, with a moderate theoretical excess of solvent, is important, but don't use greatly excessive amounts of solvent.