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You best source of information is boilerplate on the part itself.
you have that but all there is is a part number and the design voltage. 

Your next best information is the boilerplate on the appliance itself, you seem to have lost that.

So back to the part boilerplate. 

"GAL-700E-1S"  that's some sort of part number  doing a search on that suggests the brand "Galanz"  so possibly from a Galanz microwave oven.

Looking at the Galanz product line-up I find a 700W microwave oven,  This is quite possibly coincidence but engineers tend to try to make things easy to remember. so there's a good chance that this transformer if from a 700W microwave oven. so 700VA or slightly more is a good bet.

The engineers who design transformers tend to try to reduce the amount of material used as far as possible, so if if you can find another 700VA transformer it will likely weigh the same amount and be the same size.