Soft-vacuum plasma processes have been increasing in importance for high-quality, large-area, and high-rate fabrication of thin films for practical applications. In such processes, the characteristics of the deposited species (considered to be clusters) have been of interest. However, even their size has not been sufficiently investigated because of the many difficulties arising due to the high pressure conditions and the steep temperature and concentration gradients in the boundary layer, especially when the species change easily during measurements. In this work, we describe a new method of using a microtrench, to determine the mean cluster size d(c), diffusivity D, and sticking probability eta without any disturbances to the plasma or the boundary layer conditions. Using this method, the cluster size in the deposition of YBa2Cu3O7-x high-T-c superconductor by plasma flash evaporation is successfully measured as 1-9 nm, with the parameters of source powder feeding rate from 60 to 350 mg/min, torch substrate distance from 270 to 360 mm in 200 Torr, 50 kW 95%O-2-5%Ar inductively coupled rf plasma.