Add missing .size() inside an assert() to fix compilation of the debug builds

Fixes this compilation error in the debug builds of the nightlies: https://lhcb-nightlies.cern.ch/logs/build/nightly/lhcb-gaudi-head/1983/x86_64-centos7-gcc7-dbg/Rec/#show_Pr_PrVeloUT/Pr_PrVeloUT_12

WIP purely so @cattanem can decide when to un-WIP. I checked this fixes compilation, but didn't check any further.