Reply: No, there are no such limits for purchases under rate contracts.
Canada 55:
Report by the Secretariat (WT/TPR/S/249): Part III. Trade Policies and Practices by Measure: (4) Measures Affecting Production and Trade; (v) Government Procurement: (c) Preferential policies at the central government level: paragraph 232, page 109:
Regarding preferential treatment for micro and small enterprises (MSEs), the Secretariat's Report states that, "preferences could be granted on a case by case basis after an assessment of the ministry concerned."