Those who are unable to pray standing due to a disability, old age and so on, then they are permitted to pray while seated, either on the floor if that is easy, or on a suitable chair. If they are praying seated on a chair throughout the prayer, then they should have the chair such that their chest and shoulders are level and in line with the people in the row (as is normal) so long as they are not harming the sajdah of the person (or persons) behind them. That is because it is wājib to line up the rows without anyone being ahead or behind; and because their sitting is like the Qiyām (standing) of others. In this position, their legs and feet may be ahead of the line, but that is acceptable so long as their shoulders and chest are level with those on his right and left.
But if sitting on a chair like this causes harm to those behind them, and it causes them to be constricted in their rukoo’ and sajdah (or unable to make sajdah) due to the position of the chair, then the chair must be pushed forward to remove the harm from the one praying behind him. The proof for this is the principle: “The removal of harm takes precedence over bringing about good.” (Al-Ashbāh wan-Nadhā’ir of As-Subkī 1/105) And the hadeeth of the Prophet (salallāhu ‘laihi wasallam): “It not permitted to cause harm or reciprocate harm.” (Al-Muwatta of Mālik 2/745, Ad-Dāruqutni 4/228, Al-Hākim 2/57, Al-Bayhaqī 6/69, the hadeeth is hasan).
And if anyone deliberately harms the prayer of another, he is sinful. So in this situation, he must move the chair forward to prevent harming others. In this case, the back feet of the chair should be in line with the feet of the people next to him, even if that means his body is in front of the people in the row he is sitting in. That is because his chest and shoulders being in line with the row is something good, but him harming the one behind him is transgression; and “the removal of harm takes precedence over bringing about good.” And Allāh knows best.
Ref: See Al-Lubāb Feemā Lis-Salāti ‘Alal-Karāsī Minal-Ahkām Wal-Adāb, pp. 37-43.