For a toilet of less than 2 m² we recommend a radiant heater of 200 Watts.
For a toilet of 3-4 m² we recommend a radiant heater of about 300 Watts.
Note: this evaluation corresponds to a properly insulated toilet (at least from 2005) and for the most efficient heaters on the market.
In terms of budget, expect to pay €399 to €499 for premium quality products.
Examples of real projects of narrow (space saving) electric heaters for toilets – Source HeatGood Degxel
Examples of narrow (space-saving) electric heaters for WC – Source HeatGood Degxel
Examples of small extra flat 200W electric heaters for WC – Source Heat4all
Examples of small, extra-flat 300W electric heaters for toilets of 4 to 6 m² – Source Heat4all
Examples of small, slim and elegant electric toilet heaters – Source Degxel