Is it permissible for the mosque's Imam to accept a gift (a thobe) from a worshiper who had it tailored to his size?

There is no harm for the Imam or anyone else in accepting gifts and donations from congregants or others, provided it is without coveting or solicitation. Accepting it is sustenance brought to him by Allah, for the Prophet, peace and blessings be upon him, said: "Whoever receives a kindness from his brother, without coveting it or asking for it, let him accept it and not refuse it, for it is only sustenance that Allah has brought to him."