I am not recommending this instrument over the many similar instruments available from other manufacturers, but the Korg SP-200:
- measures 1328 x 285 x 122 mm
- weighs 18.5 kg
- has the full 88 keys
- strives to imitate the sound and feel of a real piano accurately
Instruments like this are designed to be a professional performance and practice instrument for real pianists, and there are no compromises here that would be harmful to a player using it as their regular practise instrument.
It's up to you whether the price, weight and size are acceptable to you.
To go smaller, lighter or cheaper, there will be compromises:
- Instruments with fewer keys mean that some pieces can't be played.
- Instruments with smaller keys should not be considered -- they will completely throw
out the player's technique.
- Doing without weighted, hammer action keys, means that the response of the keyboard will be different. This is the factor most likely to make a difference to the player's technique. Other mechanisms try to simulate weight with springs - some people find that sufficient.
The effect of this is that a player would practise getting the exact expression they want out of a piece, then perform it on a real piano and find that the sound they're producing doesn't match their expectations. This is likely to be quite a subtle effect, noticeable only by connoisseurs - but it may unsettle the player.
On the practice instrument, the action of the keys might mean that certain phrases are easier - or more difficult - than on a real piano. As an exaggerated example, imagine practising a complex Bach phrase of 16 quavers, on an electric organ with very light action and no velocity sensitivity. After half an hour's practice, you think you've nailed it. It sounds OK. Now move to a piano, and find that gentle movements that were enough to sound a note on the organ, now barely move the piano key. Or find that although you get all the notes out, they vary wildly in volume.
Do note, however, that even real pianos vary in feel, sound and response. My school's upright, my parents' upright and my piano teacher's baby grand all felt and sounded different.
It makes sense to occasionally play a different piano, just to get used to the fact that they vary. After all, concert pianists don't usually bring their own piano with them to venues.
It all depends on the player's aspirations. If they are already an experienced player, they probably know their own needs. If they intend to play on the stage, on real pianos, to fussy audiences, then absolute accuracy might be important to them. If they only ever intend to play at home, then as long as their home instrument makes a pleasant sound, that is surely enough?
Having said all this, I don't feel that even the cheapest plastic organ sold at the market toy stall actively harms a player. I suppose it might if you burned thousands of hours of playing on it into your muscle memory -- but in reality, nobody would do that.