when using Triplanar on one side for example, can I have by default the object to float and not extrude all the way? I know I can change later

The size of the object is determined by the masks on all 3 sides of the Triplanar. Currently you have masked one side only (the back side looking at the screenshot). There is no mask on the right side of the Triplanar, which means the mesh has no restriction in this dimension. If you want the object to float you need to paint a mask on the right side (or the bottom). In this case this can be a simple rectangle. Just make sure it is big enough to cover the entire height (right side) or width (bottom) of your object.

Yes, I know. I asked of there is a default setting.
I can mask or change size later on

I find the best way to start triplanar is to fill the masks and create a box. Select mask and tap the screen to mask all. Then you can carve into it with unmask, change size of limits etc.