How many linear meters is a wall 4.6 meters in length and 2.6 meters height?

Slightly more than four and a half metres long and a similar amount more than two and a half metres high.

The dimensions stated are linear dimensions. If the area is required then multiply the length by the height.

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A wall 4.6m long X 2.6 m high is 4.6 "linear metres" long x 2.6 metres high: not "slightly" different; not in fact different at all. A metre (to respect its correct French spelling) is a metre, nothing more, nothing less; and the Word "linear" is entirely superfluous because the unit is that by definition.