As noted you can't mosaic to sids in ArcGIS anymore, and I think size (number of pixels) was a major limit in the older ArcGIS versions that did provide that option. MrSid compression technology is proprietary, owned by Lizard Tech. In ArcGIS the best alternative may be to mosaic to jpeg 2000, a wavelet compression technology that is similar to MrSid but open source. ArcGIS 10 produces better jp2s than its predecessors, at least that is what I've found. You could then use that jp2 as you would a sid but if you have the GeoExpress Tools, you could convert it to a sid. If money is not a problem then buy GeoExpress with unlimited allowances of compression to sid and jp2, (although you can also buy a "cartridge" of allowable compression on an annual basis), and use it to mosaic your tifs to sid directly.
One thing that might confuse you in producing jpeg 2000 files in ArcGIS. A typical raster compression for a sid file is usually stated as a ratio 20:1. In ArcGIS the matching compression would be 5 (100/5), as it is using the term compression quality. You set these and the jp2 compression choice in the environment settings - raster storage page. If you use jp2 you can uncheck the build pyramids; these are built in to the wavelet compression file formats.
