Are there smaller units of measurement than milimeters?

Answer 1

no

* * * * *

Of course there are!

1 micrometre (or micron) = one thousandth of a millimetre,

1 nanometre = one millionth of a millimetre

1 picometre = one billionth of a millimetre

1 femto = one trillionth of a millimetre

1 attometre = one quadrillionth of a millimetre

1 zeptometre = one quintillionth of a millimetre

1 yoctometre = one sextillionth of a millimetre

and also

1 Planck's length = 162 nonillionth a millimetre (approx)