If you want to increase the volume of your longboard do you recommended going wider or longer
Going longer gives you more glide,can you get the same result by going wider?

A little of both imo, but then it really boils down to the waves your riding. Steeper the wave, narrower the board. Won't matter so much below waist high, but will make a big diff in over. Glide is about surface area, but rocker also plays a big part. Just gotta find the right balance for whatever your chasing.

I recommend thicker.
most will say no, but if you're a heavy rider you won't struggle holding a rail. It only becomes an issue in heavy waves.

Depends why your doing it.

Width will give you more volume easiest adding a half an inch over nine foot will add more foam than longer or thicker.

Width also gives you more surface area and stability.

Going a bit longer does bugger all for adding volume. It will paddle better if it is substantially longer but thats not volume and it would have be more than a few inches to notice.

Thickness can make them harder to sink. But as cobra said only if you are a lightweight, this can be over come with deck roll and low rails, Thicker and narrow can paddle really good and narrow boards go rail to rail better.

But really volume and longboard should be a non issue imo, there over 9ft long amd 22 wide you shouldn't be worried about volume.

Its more likely a fitness, positioning or paddling issue, not a volume issue of your struggling to catch waves.

Rocker rails and outline will do more for a board than making it marginally wider longer or thicker

I can only speak for myself: My boards get longer as I get older, wider and thicker.