Generally an electrical convection heater of the normal size is enough to heat a normal log cabin. We used to work from a 58mm cabin through all the seasons and it was more than enough to heat a 6m x 3.5m space.
You can though also consider underfloor heating which is very popular these days and of course wood burner / wood burning stove. For this though you will need to seek the advise of a qualified installer.
We would not recommend a gas heater in your log cabin without adequate ventilation due to gasses being built up.
It is always a good idea to install a carbon monoxide detector if you are using a gas heater.
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