Make a list of your measurements like this:
<TABLE style="WIDTH: 48pt; BORDER-COLLAPSE: collapse" border=0 cellSpacing=0 cellPadding=0 width=64 x:str><COLGROUP><COL style="WIDTH: 48pt" width=64><TBODY><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #f0f0f0; BORDER-LEFT: #f0f0f0; BACKGROUND-COLOR: transparent; WIDTH: 48pt; HEIGHT: 12.75pt; BORDER-TOP: #f0f0f0; BORDER-RIGHT: #f0f0f0" height=17 width=64>1 1/2"</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #f0f0f0; BORDER-LEFT: #f0f0f0; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #f0f0f0; BORDER-RIGHT: #f0f0f0" height=17>2"</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #f0f0f0; BORDER-LEFT: #f0f0f0; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #f0f0f0; BORDER-RIGHT: #f0f0f0" height=17>3"</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #f0f0f0; BORDER-LEFT: #f0f0f0; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #f0f0f0; BORDER-RIGHT: #f0f0f0" height=17>4"</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD style="BORDER-BOTTOM: #f0f0f0; BORDER-LEFT: #f0f0f0; BACKGROUND-COLOR: transparent; HEIGHT: 12.75pt; BORDER-TOP: #f0f0f0; BORDER-RIGHT: #f0f0f0" height=17>5"</TD></TR></TBODY></TABLE>
name it List and assign it to Data Validation in B1. Define the following names for your second choices:
_1_1_2
_2
_3
_4
_5
and for your dependent validation use the formula:
=INDIRECT("_"&SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(B1," ","_"),"/","_"),"""",""))