As the property was inherited by your grandfather, the same comes within the ambit of coparcenary property. Within the joint family there is a narrower body called the Coparcenary. This includes the eldest male member + 3 generations. For eg : Son – Father – Grandfather – Great Grandfather. This special group of people are called coparceners and have a definitive right in ancestral property right since the moment of their conception. They can get their share culled out by filing a suit for partition at any time. A coparcener’s interest is not fixed it fluctuates by birth and deaths in the family. Coming to the questions you have asked:
1. Yes you can challenge the said sale on proving that you were unaware of the said sale till the filing of the suit in the court of law and that when the sale deed was executed, permission of all the coparceners was not taken including you (for a minor, a permission has to be taken from the Court before parting with the coparcenary property), you can claim your share in the land. Your grandfather had to take the permission of all the coparceners before selling the property and if the same was not taken, it can be challenged. however, we would like to inform that in cases of legal necessity/benefit of estate the karta can alienate joint family property. However such an alienation can be challenged by the continuing coparcenors as not being for legal necessity or benefit of estate within 12 years of knowledge of sale/gift.
2. No there is no such limit with respect to the salary of individual or a family for owning an agricultural property. however there are laws governing the total agricultural land that can be owned by one individual or family and this varies from state to state.
3. A minor can claim his share in the ancestral property after attaining majority if the permission from the Court was not taken with respect to the minor's share and proving that you were unaware of the said sale. You being a minor can file the case through your next friend for your share and protect your share in the Ancestral Property.