Why doesn't cyclohexene have a cis-trans isomer whereas cyclodecene has one? Is strain a factor?
You simply can't make a transoid molecule of cyclohexene because there is not enough "length" in the carbon chain.
I don't know if it is "strain" per se--but a putative transoid cyclohexene would have extraordinarily exotic bonding arrangement--thus being very high in energy.
Yes, in cycloalkenes with of 7 or less carbon atoms, the high ring tension is a prohibitive factor, but in higher cycloalkenes it's not that great so they can show the geometrical isomerism.