When I have done stitch in the ditch quilting - my largrst piece was about 70x90 inches -
The machine I used was a Pfaff 1471 - mid 1980s model - with the built in walking foot
I safetypinned about every three to four inches
I started at one edge and stitched straight across to the other edge.
I stitched ALL the lines only going one direction - it got a little bulky towards the last line - but I feel that if there is going go be any wandering - I want it to all go the same way.
Then I tuyned it and went the other direction.
I stitched the long way first - less bulk in the harp for the first pass.
It is essential that the layers be smooth before starting to sew.
I pin the layers together on a large table. Something that helped me - the backing fabric should not be pulled tight as a drum - because when it is relaxed. It draws up the batting and top - and then the top wants to creep a bit and/or pucker.
I like the look of SITD quilting.