In a large enameled, glass or other non-reactive saucepan, combine 6 cups water, cider vinegar, sugar and salt. Bring to boil, reduce heat to low, and then simmer mixture while proceeding with recipe.

Place a third of the dill in bottom of jar. Top with a third of the onion and garlic. Starting with smallest cucumbers, pack 1 layer horizontally over garlic. Place another layer crosswise over first. Continue until jar is two-thirds full, packing jar tightly and filling in gaps with cucumbers halved lengthwise.

Add another third of the dill, followed by another third each of onion and garlic. Top with cucumbers, packing tightly. Top with remaining dill, onion and garlic.

Place jar in large plastic basin, and carefully pour in hot vinegar mixture (brine) into jar to fill it completely. Using long wooden skewer, gently nudge the pickles deep in the jar to dislodge any air bubbles. Add more brine, if necessary, and allow mixture to sit for 5 minutes. Add more brine to fill jar to brim, and top with mustard seed. If using glass lid, place lid directly on jar. If using metal lid, to prevent corrosion place large piece of wax paper over top of jar before fastening lid.

Allow mixture to cool to room temperature. Remove jar from basin, and wipe clean with damp sponge. Refrigerate jar for at least 24 hours before eating. The pickles may be refrigerated for up to a year.