Very small labels and components in Mapbox

I am trying to create a navigation application using Mapbox GL JS. But the text labels are too small along with the searchbox. Where in the Mapbox GL JS API can we set the tilesize and ppi to fix it?

The best way to adjust the size of the text labels is to create a custom map style using Mapbox Studio, rather than programmatically manipulating the size of the map labels using the GL JS API (unless for some reason you need the text label size to be responsive on the client side).

You can navigate to studio.mapbox.com, select the New style button, choose a template to customize, and then click the Customize button. This will open the map editor interface, where you can navigate to the Layers panel on the lefthand side of the interface and select a layer whose properties you would like to edit.

For example, in the screenshots below I selected the state-label layer and changed the Text size property from 18px to 30px. More details about styling layer properties can be found in the documentation here.

To persist these changes to the map, you can publish your style, as described in the detailed documentation here. The way in which you can then add the custom map style to your application is dependent on the application you are developing for. The linked documentation specifies the appropriate steps for web, Android, iOS, Unity, and several other third party options.