Many labs can do that, but its going to be pricey with all the extra work to get a nice image at larger sizes. I have done some of this, most flatbed scanners will work just fine, scan in the highest native resolution into 16bit TIFF, then do a little adjustment in PS. I think I was scanning at 600ppi, so with that size file a scanned 4x6 should be able to print at the sizes you want, maybe be a bit softer though. I think It might be worth trying a few yourself, order a coupple prints see what kind of results you get, I think with just a little work you will see som pretty good results, if not then try a lab that can to drumscans and make some adjustments.