She can pass 1mm if they sit in the right place. The vets have a few alternatives. Some options we've had are:
1. Wait and See (like what you're doing) with effective painkiller (eg 1kilo pig - 0.2ish mls 'dog' metacam (1.5mg/ml) - twice a day)
2. If it's wedged in the urethra they can sometimes fish out with forceps in a sow (Snowflake's was 6mmx4mm and all spikes!). Unpleasant but quick.
3. A fluid bolus injected under the skin (volume between 5 and 10 mls so it looks like a massive lump) coupled with - in our case - an injection of opioid painkiller. Poor Ivy was totally out of it for quite a few hours which was very upsetting to see. She also peed like a tap. If she passed her stone I didn't find it but it was gone on the next x-ray. We'd gone in the first place because she'd suddenly squeaked and peed a small puddle of what looked like neat blood so we think there was definitely an initial stone passed naturally.
4. Surgery.
In Ivy's case when I looked at the x-ray I wasn't entirely convinced it was a stone. I asked if it could be an accumulation of sludge in the tube where the urethra leaves the bladder and they said it might be. I only thought that because it seemed to be a small and neat 'bullet' shape as opposed to Snowflake's ninja death star. The fluid bolus blew her tubes clear anyway.
Anything which makes them pee less or hold in the pee restricts the flow and sludge can accumulate making it more likely to get stones forming. UTI makes them hold it in because it hurts to pee so prompt treatment of UTI with antibiotics is good - both my girls were effectively treated with antibiotics for UTI. Snowflake didn't get another stone - or if she did she didn't tell me - and she passed when she was 5. Ivy also didn't get another although we tried to get her to drink more by putting water bottles pointing in the hidey where she slept so she didn't have to come out in the open to drink when it was dark... this worked for her and she often woke me at night with her teeth on the water bottle. Their cage mates didn't get stones and were presumably eating/drinking the same things.
Baytril tastes grim... fortunately metacam painkiller is delicious and they will fight for the syringe!
My vet said she fed bell pepper to help prevent formation of calcium stones but also said it wasn't known for sure... just rumours. Snowflake had been getting a lot of celery which some people are wary of, so we began to avoid this and to give a good lump of bell pepper each day (about 1/4 to 1/8th depending on size of pepper). We also started to give 'soft' bottled water - our water area is so hard that even filtered water was higher calcium than the bottled we found. Look for Ca on the back of the label and google your water company - some areas of the UK actually have soft tap water anyway (you can put your location on the info bit on the LHS of the screen if you like). Watery veg like cucumber increases fluid intake. Wash veg before they get it to make it nice and wet. Keep 'them peeing! Good luck tiny girl
x