Is it from a business seller? Some private sellers do not accept returns and even if they do will only accept them if there is a problem with the item.
You cannot expect them to send you the item for you to try on & then when you decide it's 'not for you' pay for you to return it.
I know catalogue Companies do this but they can afford it, private sellers would be seriously out of pocket if they did this.
Contact the seller through ebay and ask them for the chest measurement, length etc. (I always put those on my listings, just in case). As the others say, if the seller accepts returns, you'd still have to pay to send it back - but their returns policy should be listed in the description (I know mine is).
Its a risky business buying a coat on ebay and one of the disadvantages. I have bought things successfully but a couple more that have been lovely but not fitted. Luckily they were low cost and I gave them to the charity shop, but you could always resell the coat on ebay if it doesn't fit.