I've aways used 15" marshal town. You'll pay £40 ish but itll last a long time. Dont buy cheap as they wont last long, and cant take the punishment that a trowel gets, rivets snap etc.
Take my advice, spend money now on what ever flavour, just make sure its a decent make.
I would also go for a Marshalltown or a Tyzack pre-worn in trowel if you are just starting out. But I would get a 12" one,it will help you get the feel of the trowel and once you have got used to it and done a few walls and you are comfortable with your standard of work be it cement rendering or skimming you will know when it is time to move up to a larger trowel. Also make sure the trowel is a stainless one a bit more dearer but well worth it if you are only skimmin'. Years ago before you could get stainless our trowels used to go rusty with lots of skimmin work but the rendering used to bring the shine back on it but we had to "break" our trowels in before use but now its all done for you...
I've got two 11" and one 13". I would reccomend the smaller one if you are just starting out and get yourself a spat for flattening. Something between 16-20"
Edit to say, marshalltowns everytime
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