We consider high spin, $s$, long twist, $L$, planar operators (asymptotic
Bethe Ansatz) of strong ${\cal N}=4$ SYM. Precisely, we compute the minimal
anomalous dimensions for large 't Hooft coupling $\lambda$ to the lowest order
of the (string) scaling variable $\ell \sim L/ (\ln \mathcal{S}
\sqrt{\lambda})$ with GKP string size $\sim\ln \mathcal{S}\equiv 2 \ln
(s/\sqrt{\lambda})$. At the leading order $(\ln \mathcal{S}) \cdot \ell ^2 $,
we can confirm the O(6) non-linear sigma model description for this bulk term,
without boundary term $(\ln \mathcal{S})^0$. Going further, we derive,
extending the O(6) regime, the exact effect of the size finiteness. In
particular, we compute, at all loops, the first Casimir correction $\ell ^0/\ln
\mathcal{S}$ (in terms of the infinite size O(6) NLSM), which reveals only one
massless mode (out of five), as predictable once the O(6) description has been
extended. Consequently, upon comparing with string theory expansion, at one
loop our findings agree for large twist, while reveal for negligible twist,
already at this order, the appearance of wrapping. At two loops, as well as for
next loops and orders, we can produce predictions, which may guide future
string computations.Comment: Version 2 with: new exact expression for the Casimir energy derived
(beyond the first two loops of the previous version); UV theory formulated
and analysed extensively in the Appendix C; origin of the O(6) NLSM
scattering clarified; typos correct and references adde