What if you have a single product that comes in various sizes? Create a dynamically generated select box using Ditto:
<select> [[Ditto? &parents=`id of container` &tpl=`Foxycart-select` &display=`all`]] </select>
Where your template chunk Foxycart-select looks something like this (line breaks added for legibility):
<option name="size" value="[+ItemSize+]{p:[+ItemPrice+],w:[+ItemWeight+],c:[+PartNo+]}"> [+ItemSize+] </option>