Plastering trowels come in all sizes and materials, but generally a stainless steel 11"/280mm is a good size trowel for general plastering. These will cost you from around £20 and upwards.
Lots of beginners go out and buy the biggest trowel they can find, which will be cumbersome to use and give a poor finish. Larger trowels are good for finishing, but an 11'/280mm - 12'/305mm is the best size for applying first and second coats of plaster.
Use your older trowels for applying backing coat plasters or sand/cement render as these will damage your finishing trowel.