If you want to use a medicine cabinet I would probably stay with the 24 inch and use side lights. You'll need to keep in mind that a bar light above the medicine cabinet will need to extend enough to clear the depth of the cabinet, so finding such a light may be difficult. There are cabinets that have built-in lights so you may want to look at those. Check these out. If you choose a cabinet with built-in lights, I'd go with a wider than 24" cabinet. Other option: keep the light fixture you have and put in a framed, or unframed mirror that is a little wider than the lights. Here are some visuals.

I think different approaches work for different people. Our bathrooms are small (master is 6' x 10', hall bath is 6' x about 12'). In the master bath, we have a 60" vanity that abuts the wall on one side. Over that vanity, we have a full-size mirror (as described by palimpsest) with a 24" LED fixture above each sink coming through the mirror (love that look). In the hall bath (used by DD, and also serving as guest bath), the vanity is in a 55" alcove. We chose to use a free-standing 48" vanity and a "free-standing" decorative mirror mounted horizontally, with the same LED lights hung vertically to the left and right of the mirror. The light from the LEDs (George Kovacs) is terrific, much much better than the light we got from the recessed lighting and over-mirror lights in our previous home.

Many people choose vanity lights that are about 75 percent the width of the mirror and center the light above the mirror. You can also choose to use a bath bar light that is exactly as wide as the mirror. An LED mirror is another choice. https://www.lightsonline.com/learn/lighting-ideas-by-room/bathroom-light-fixtures-guide 

You can have mirror cut to any size by a glass shop, but if this is a bathroom that is used every day for getting ready in the morning (not the powder room), if I were you I would engage an electrician to have sconces on either side of a smaller mirror. The light you get from sconces is much, much better than overhead lighting. This isn't a big job and if you pay for it once you'll have it forever. A decorative mirror will look better than a big mirror too. If you do go with a big mirror, frame it in some type of molding so it doesn't look like a tract home mirror.