Is a wife sinful, Islamically speaking, if she refuses to have more children after having six, and is her husband entitled to force her to do so?

Procreation is a right shared by both spouses, and neither of them is permitted to prevent the other from it except for a legitimate, recognized Sharia-compliant excuse. A large number of children, or the old age of some of them, or the wife's age, or her being in a foreign land, are not considered justifications for prevention. If there is a legitimate benefit in delaying procreation temporarily, there is no harm in the spouses reaching an understanding and agreement on that, and it is permissible if they both consent.