Hi,
Neither the Ohm value nor the size will show what wattage it can stand.
Oval speakers are often used in old music devices (1960ies). Is this the case with your speaker?
Whether or not ... it does not make much difference.
I assume it can withstand a couple of watts. Maybe 1..10W true RMS Watts.
You may use a 0.1W amplifier or a 250W amplifer. It will work for normal sound level.
With a 0.1W amplifier and increasing sound level, you will hear distortions caused by the amplifier's limit (clipping)
With a 10W amplifier it will depend on the type of music (frequency, frequency range) what will limit the sound level vs sound quality.
With a 250W amplifier it will be the speaker that is the sound quality limiting device.
In either case: When you hear the sound becoming distorted it is a good reason to reduce sound level.
Klaus