boy o boy have i had some trouble with my engine mounting bolt?

i was told it was 16mm . and i tried a 16 and it didnt fit???
i tried hammering it onto the bolt and still no joy .
15 was way too small 16" was small
17 was too big ??
anyhow after about 1 hour of swearing and a lot of thought i magaged to move the blasted engine mounting bolt , only to find it didnt come loose it just span on its axis













?
so i decided to take off the two bolts one on either side of the engine mount, just set back from the mount, and hey presto the engine is now detached from the gearbox
however i now just have to take the back off the car for easy removel of the engine . as lighter it might be but still too heavy for me to lift
are the bolts on the rear left/right had side bumper panel 13mm ??
as mine are well rusted and long bolts?
so guess i need a long socket shaft etc?
anyhelp tonight would be great
please get in touch
regaurds
richard