During the marking operations planning in forest ecosystems, designing the canopy gap size (CGS) is not clearly possible and a marker can only be capable of calculating the expanded gap size (EGS) that will be created by cutting and felling trees. Therefore, the main goal of the current study was to provide a solution to this problem on the basis of accurate prediction for CGS, before applying selection methods, through developing regression models for predicting the CGS in Glandroud forests. On the basis of full inventory for measuring the area of canopy and expanded gaps using radial technique, calculating the species diversity indices and observations of physiographic units, the regression models were developed for estimating the CGS as the response in the research. The results showed that the site fraction occupied by expanded gaps has one hectare more than the fraction occupied by the canopy gaps. Furthermore, the results of correlation tests showed that the CGS had significant relationship with the Shannon diversity and species dominance indices. The results of analyses indicated that multiple linear regression log-transformed from the power function including EGS, correlated species diversity indices of gaps surrounding trees predicted the responses with statistically acceptable certainty and accuracy.