I think this is not possible with getResources
But its possible with pdoResources (included with the pdoTools - package) like that:
[[!pdoResources?
&parents=`2`
&limit=`50`
&tpl=`product`
&showHidden=`0`
&includeTVs=`size,price`
&sortby=`{"size":"ASC","price":"ASC"}`
&toPlaceholder=`results`
&hideContainers=`1`
]]
your TVs should be of type number to be sorted correctly
the TV's input-type should be 'Number' (check it in the input-options of your TV)
pdoTools (pdoFetch::addSort) does automatically CAST to DECIMAL, when the type of your TV is 'Number'