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In this paper, we derive the closed form analytical solutions for the effective viscosity of the suspensions of solid spheres that take into account the size effects. This result is obtained using the solution for the effective shear modulus of particulate composites developed in the framework of the strain gradient elasticity theory. Assuming incompressibility of matrix and rigid behavior of particles and using a mathematical analogy between the theory of elasticity and the theory of viscous fluids we derive the generalized Einstein's formula for the effective viscosity. Generalized Brinkman's solution for the concentrated suspensions is derived then using differential method. Obtained solutions contain single additional length scale parameter, which can be related to the interactions between base liquid and solid particles in the suspensions. In the case of the large ratio the between diameter of particles and the length scale parameter, developed solutions reduce to the classical solutions, however for the small relative diameter of particles an increase of the effective viscosity is predicted. It is shown that developed models agree well with known experimental data. Solutions for the fibrous suspensions are also derived and validated.