import re
text = '''5.3 x 2.5 cm
11 x 11 mm
7 mm 
13 x 12 x 14 mm
13x12cm'''
x = "(?:\.\d{1,2}|\d{1,4}\.?\d{0,2}|\d{5}\.?\d?|\d{6}\.?)"
by = "(?: )?(?:by|x)(?: )?"
cm = "(?:mm|cm|millimeter|centimeter|millimeters|centimeters)"
x_cm = "(?:" + x + " *(?:to|\-) *" + cm + "|" + x + cm + ")"
xy_cm = "(?:" + x + cm + by + x + cm +"|" + x + by + x + cm +"|" + x + cm + by + x +"|" + x + by + x + ")"
xyz_cm = "(?:" + x + cm + by + x + cm + by + x + cm + "|" + x + by + x + by + x + cm + "|" + x + by + x + by + x + ")"
m = "{}|{}|{}".format(xyz_cm, xy_cm, x_cm)
a = re.compile(m)
print a.findall(text)