Re: is there a "standard" electric guitar strings size?
I would estimate that about 90% of new 6-string guitars come from the factory with 9-42.
Sune, you should know that there is no "right" or "wrong" string gauge for any guitar - it's all based on what you prefer in terms of feel and tone. It's good to try different gauges at first to find out what fits you the best. All my guitars are tuned to a drop-C tuning with 11-50 gauge strings, and I've only come to that perfect balance for me after a bunch of experimentation.
Since you're a beginner, lighter strings are probably going to be easier for you to play until you get your fretting hand built up with permanent callouses and finger strength. I would advise buying 9-42 gauge strings even if that's not what your guitar came with originally - if it had 10's on it from the factory, the 9's will feel slinkier and more forgiving on your hands.
Good luck, and stick with it! Playing guitar is one of the defining aspects of my life, and I've gotten years of pure enjoyment from it.