As far as I know Wiltshire has always been roughly the size it is now. The first of the boundary adjustments occurred in 1844. I have an old atlas dated about 1840 which shows outlying bits of Wiltshire existing as islands in other counties. Kingswood near Wootton-under-Edge (now in Gloucestershire) was in Wiltshire at that time, and there were small chunks in Berkshire, one near Newbury and another as far away as Windsor which is at the far end of Berkshire! These remained with Wiltshire by accidents of land ownership and the most bizarre of these were cleaned up in 1844, the odd bits being passed to their respective counties.
This was when Minety (Glos.), which had been entirely surrounded by Wiltshire parishes, was transferred over to Wiltshire. In the 1851 and later censuses you will sometimes see that someone was born in "Minety, Glos." This is not an error, as it was indeed in Gloucestershire, at the time the person was born.
At the time of the 1891 census, the period which I know best, various parishes on the edge of Wiltshire were still split between two counties. Shalbourne (partly in Berkshire) and Stourton and Maiden Bradley (partly in Somerset) spring to mind. These were transferred completely to Wiltshire in the mid-1890s. There were also parishes which were taken away from Wiltshire at about the same time. About eight parishes at the southeastern edge of the county were given to Hampshire, including Bramshaw, West Wellow and Damerham. Damerham has actually given its name to one of the old hundreds of Wiltshire but is itself no longer in the county.
In 1897 Kemble, Poole Keynes and Somerford Keynes in the northwest were transferred to Gloucestershire, so Wiltshire lost about ten parishes in the 1890s while gaining a few bits and pieces.
In 1930 there was another small boundary adjustment, and Ashley and Long Newnton were also transferred to Glos.