The extraordinary biodiversity of tropical regions has attracted many researchers. Despite an increasing number of genome size (GS) estimations, those of tropic flora remain poorly explored. In this study, we applied the glycerol-preserved nuclei protocol for GS estimation in the field conditions of a tropical rainforest in Brunei Darussalam, northern Borneo. Samples were prepared in the field following this protocol and subjected to the flow cytometry analysis in the laboratory approximately 1 month later. The glycerol-preserved nuclei protocol enabled us to perform GS estimations of thirty taxa of Euphorbiaceae s.l. (twenty taxa of Euphorbiaceae and ten taxa of Phyllanthaceae family, respectively), all representing first estimates of GS for respective taxa. We found 5.09-fold overall variation in GS, with the lowest value in Croton sp. (2C = 0.97 pg) and the highest value in Aporosa elmeri (2C = 4.94 pg). The vast majority of species presented very small GS (77%), while 23% of the species had small GS. Using available data of Euphorbiaceae s.l. from the Plant DNA C-values database, we also tested the correlations of GS with climate (tropical vs. temperate) and growth form (woody vs. herbaceous). Tropical taxa of Euphorbiaceae s.l. (incl. Euphorbiaceae s.s. and Phyllanthaceae) have significantly smaller GS values than temperate ones. No significant difference in GS between woody and herbaceous taxa was detected.