This paper presents a sequential dimensioning methodology for frequency restoration reserves in a multi-area power system based on chance-constrained optimization. In the first stage, the reserves to handle the reference incident in each area are dimensioned. Then, the transmission network usage for providing these reserves is calculated and the remaining transfer capacity is used in the next stage where the reserves to handle normal imbalances are dimensioned. The optimization problem in each stage seeks to allocate reserves such that the total volumes of reserves and the line flows are co-optimized. Reserves are dimensioned for four seasons instead of the current static approach with yearly dimensioning. By adjusting the reserve requirements based on the system’s needs, the total reserves are reduced. Also, the results demonstrate high potential in sharing reserves among bidding zones in the Nordic synchronous area.