Make sure the i spector panel is open in pcblib. You can change all the criteria of elements there.
Seleectt he pads you want to modify and simply change the parameters there.
As for precise positioning.
I dont bother with guidelines and measuring.
Just set the grid to the correct pitch.
Look at the bottom. Altium shows you absolute xy coordinates and relative coordinates (like when drawing a trace). I simply draw the traces on a mechanical layer.
Lets make a component , 'the free_electron way'
To begin we need four pads 1.27 mm apart.
I place my first pad somewhere. I dont care where. Just place it.
Edit -set origin -pin. And click on the center of the pad. Make sure snap to object centers , and snap to grid is on in the snap control lanel (bottom right of the screen is a little tab that says snap)
Now. We have our first pad and the coordinate system is centered on there.
G grid. Type in 1.27mm and hit enter.
Place three more pads on the grid. Click click click.
Now, we need a pad 4mm above these and centered between the middle two of the four we just placed.
G grid . 1.27 halved is 0.625 so key in 0.625.
E set reference to position. And click on the gridline centered between two middle pads.
G 4mm
Go one gridline up. Bang place pad.
Essentially you use the set origin function to define the starting point and you use the grid function to pick the correct offset. Be ause altium 'snaps' to objects and grids the creation process becomes extremely simple.
If you have a step file it is even easier. You go in 3d space, place markers on objects, flick to 2 d mode and you placement will snap to the markers. No measurements required.
Leave all the calculations to the machine