Is it okay to have a big expensive wedding as a Muslim?

It depends. Celebrating a wedding is a sunnah, and spending on it isn't haram in itself. But Islam warns against waste, showing off and going into debt. A simple, joyful wedding that's within your means is the ideal. A big wedding is fine if you can truly afford it and it stays free of haram elements.

Weddings should be celebrated

Islam wants marriages to be public and happy. When a companion got married, the Prophet Muhammad ﷺ told him: "Give a banquet, even if with one sheep" (Sahih al-Bukhari 5167). This wedding meal, the walima, is a sunnah. Music with the daf and joyful gatherings are also part of the tradition, within Islamic limits.

The problem is excess

وَكُلُوا۟ وَٱشْرَبُوا۟ وَلَا تُسْرِفُوٓا۟ ۚ إِنَّهُۥ لَا يُحِبُّ ٱلْمُسْرِفِينَ "And eat and drink, but be not excessive. Indeed, He likes not those who commit excess." Quran 7:31

Spending becomes a problem when it's:

Funded by debt, especially interest-based loans or credit cards.
About showing off or competing with other families.
Wasteful, with huge amounts of food thrown away.
Pressure on the groom or bride's family that delays marriage for years.

Invite the poor too

A companion reported: "The worst food is that of a wedding banquet to which only the rich are invited while the poor are not invited" (Sahih al-Bukhari 5177). If you're spending, make sure some of the blessing reaches people who need it.

Keeping it halal

No alcohol, even if non-Muslim guests expect it.
Modest dress and, for many families, separate spaces for dancing. See Can I still dance at weddings as a Muslim?
Plan around prayer times so guests can pray.

A tip for converts

Many converts feel pulled between a big Western wedding their family expects and a simple mosque nikah. You don't have to choose extremes. A small nikah followed by a modest reception that your family can attend is a beautiful middle way. Put money into your marriage, not just the wedding day. For the mahr, see What is mahr and how much should it be?

Key points

Celebrating with a walima is a sunnah.
Spending is fine within your means.
Avoid debt, waste and showing off.
Include the poor and keep it free of haram.

Tip: Set a wedding budget together before planning anything, and stick to it. Starting married life debt-free is worth more than one extra-fancy evening.