import re

regex = r"(?<!\S)(\d+\.\d+x\d+\.\d+x\d+\.\d+) [cm]m(?!\S)"
test_str = "The study reveals a speculated nodule with pleural tagging at anterior basal segment of LLL, measured 1.9x1.4x2.0 cm in size"

print(re.findall(regex, test_str))