Match the eye to the thread
The thread should pass through the eye without being crushed or repeatedly scraped. If threading is difficult or the strand frays at the eye, change the needle before changing your tension.
A larger eye may help with a full sashiko thread, but the shaft still needs to suit the fabric. Check the hole left in a spare area rather than choosing by eye alone.
Match the point to the layers
A sharp, stable point helps cross denim and patches, while a less aggressive path can be kinder to a loose or delicate weave. Avoid forcing a needle that catches every few stitches.
Fold the project as it will be sewn and test the thickest place. The usable size is the one that moves through that area without bending, dragging or distorting the cloth.
Let comfort guide the final choice
After a short swatch, check your grip, stitch rhythm and the condition of the thread. Hand fatigue is useful evidence that a different length or eye may be needed.
Record the needle with the thread and fabric in your project notes. Repeating a successful pairing is faster than starting from a generic size description each time.