Thermal lens calorimetry has several attributes that make It suitable for small volume detection, Including high sensitivity and use of focused laser excitation. This work considers the fundamental trade-off between detection volume and sensitivity, which Is related to divergence of the focused beam. A criterion for minimum sample volume for a given path length Is derived. Sources of noise which depend on how tightly the laser Is focused through the sample are considered. Sensitivity and precision of thermal lens measurements made on 1 mm path length, smaller volume and 1 cm path length, larger volume samples are compared.