In the first part of this work, the development of a capital cost optimisation model for sizing three phase separators was described. The developed model uses GRG Non-linear algorithms to determine the minimum cost associated with the construction of horizontal separators subject to four sets of constraints. In the second part, an experimental test rig was designed and used to investigate the effect of gas flowrate, liquid flowrate, and slenderness ratio (L/D) on separation performance of horizontal three-phase separators. The results obtained indicated an inverse relationship between increase in gas and liquid flowrate and the separator outlet quality, and a direct relationship between increase in slenderness ratio and separator outlet quality. The results also showed that the gradient change of the percentage of water in oil outlet with respect to slenderness decrease up to ratios of 6:1 hence separation rate increase. Above ratios of 6:1 the separation still increases but the gradient change in separation drops off implying the benefit in terms of separation is diminishing beyond this point. Therefore, optimum ratio for technical reasons is 6:1 slenderness ratio.