I've heard rumors of a 5" hydrometer, but the links are all dead. I had a similar question a while back:
I'd love to get my hands on something I could drop in a gallon carboy of wine and leave it in there. No luck yet. I've been pulling samples with a sanitized turkey baster and using a sanitized sample column with a standard hydrometer, then pouring the sample back in. I really don't like doing that but I haven't had any contamination problems yet. If it is still early in the ferment and it is a high gravity wine I might get lucky and be able to put my hydrometer into the carboy as it floats high, but when it attenuates down to less than 1.000, no chance.
The other alternative is a refractometer, but I make do with pulling samples carefully and using a hydrometer.