JOEL - There are no church-wide regulations or standards for Sunday dress. Much of what determines the proper attire will depend on the time and place where you are living. If you look at saints over the entire world, dress standards will vary quite differently according to the time and culture.
Wherever you live, what you wear on Sunday should reflect a sense of modesty and respect for a house of God. We go to church to worship God. How we present ourselves reflects how much respect we have for Him and for those with whom we worship.
With our God-given free agency, we make a choice about things like this, based on our faith and understanding, recognizing that we are accountable to Him and to others whom we influence. Therefore we should avoid dress or manner which calls attention to itself and symblizes either rebellion or nonconformity to the values of modesty, humility, decency and propriety. If most of the women are wearing a dress in your ward on Sunday then that is what you should wear. Look to the example that is set by your ward leaders for guidance.
Of course, we should never judge people according to what they wear on Sunday. There may be circumstances that we are not aware of that determines what a particular member is wearing. Occasionly an investigator might come to church wearing pants, simply because she might not own an appropriate dress. Of course, no one would turn her away. It is more important that she is there, regardless of what she is wearing.
Generally speaking, for other church related occasions, the standards of dress are based on both modesty and appropriateness. For example, shorts that would be worn appropriately while playing sports such as a church basketball game would be inappropriate for attending a Young Women's or Relief Society activity.
In the Church pamphlet, 'For the Strength of Youth', the youth are told;
“Immodest clothing includes short shorts, tight pants, and other revealing attire. Young women should refrain from wearing off-the-shoulder, low-cut, or revealing clothes. Young men should similarly maintain modesty in their dress. All should avoid tight fitting or revealing clothes and extremes in clothing and appearance.”