I worship four Buddhas and Bodhisattvas: Shakyamuni Buddha, Amitabha, Guan Yin and Maitreya.
The Amitabha statue is longer and is placed in the center. Shakyamuni Buddha is to his left and Guan Yin is to his right. Maitreya is on the altar. They are of different dimensions.
Is this arrangement appropriate?
Buddhas and Bodhisattvas certainly do not mind how their statues are arranged.
Any arrangement is acceptable as long as it looks good to you.
More importantly, keep the altar adorned to the best of your abilities. The best way to adorn and worship them is to purify your mind and keep the Buddhist precepts.