I've discovered a couple of things that should make life easier.
Ok, if you have a map, set your grid size to whatever you like, so if the map size is whatever, set the grid to 10 x 10 for 10 feet squares.
When you insert a symbol, probably raster, from the palette it'll drop in at whatever size it is. Now I don't understand how that relates to the "map canvas". However, if you have snap turned on, you can drag the symbol to a corner of a square. You then resize it by reselecting the object and dragging the handle until it snaps to the square.
The next time you place this same object FM8 will remember what size it is and it will fit the grid, but it is placed via its center point.
This is a problem because it is not natural. I can't even get close to having it drop into the square.
I then have to re-select each object and drag the object into position on the grid.
Workable... but it means making a drawing to use as a template and sorting out all the graphic tiles you plan to use beforehand. It would be more natural if the object placed was inserted by its top left corner.
However, I found another approach, which is to select the object from the palette and then click on the intersections of the grid and drag to the opposite corner, see the dashed selection below, which then places the object accurately into the grid. I have all the snapping features turned on.
And good morning!