How to position a part relative to another parts size/length?

I tried searching on google to no avail.

Help much appreciated!

You have not provided enough information on what you actually want to achieve.
Can you please provide further information? If needed, you can even do a sketch in mspaint.

so i want to make a viewport particle emitter, but first i need to get the length of a part and position another part based on that length (hopefully that made sense.)

In what way should it be positioned relative to the other part? (a picture/drawing may help)

Like this, i can get it like picture 1, but not like picture 2 (and sometimes picture 1 dosen’t work.)