Hi
I've been reading your posts below and it reminded me of something . When I first put my 18" female with my 21" male , I found them copulating next morning ... That was in march . In july my male was measuring 25" and my female 19" and there were still no sign of eggs . It took several months ,(december) before she finally laid her first clutch of eggs , even if they mated several times during that period . She was then measuring 25 inches and had no problems with egg laying...
Another thing , my male water dragon mated a few months ago with another female , who never laid eggs before . She is a full grown female , and even after copulating with my male , she didn't start producing eggs ...
It doesn't mean that if you put your female with your male , she will automatically start producing eggs and , maybe , become egg-bound . And your female could also be incapable of producing eggs yet , as she is still a bit young .
Ok , your male might be slightly bigger than mine was at the time they first met , but I don't think he would be a threat for your female . And remember , it can take several months before she lays her first clutch of eggs and your male will look even bigger compared to her when you finally decide to put them together .
This is your decision , but I thought I would give you my opinion . My lizards have been together since that very first day they mated , and I never had any problem .... Well just one ... I separated my female recently because once she started , she never stopped...She laid clutches after clutches so I thought I would give her a break (the time it takes to reduce their daylight to 8 hours per day (to stop the breeding season). You should see my poor male when he sees his girl running around in my room , it drives him crazy ... 
Good luck
Sarah
). so im sorry it took long to thank you and reply. but back to your advise i think im going to take it when tethys hits the 20" mark shes 19 1/2" right now so ill give her another 2 weeks or so.