  We are developing large TES arrays in combination with FDM readout for the
next generation of X-ray space observatories. For operation under AC-bias, the
TESs have to be carefully designed and optimized. In particular, the use of
high aspect ratio devices will help to mitigate non-ideal behaviour due to the
weak-link effect. In this paper, we present a full characterization of a TES
array containing five different device geometries, with aspect ratios
(width:length) ranging from 1:2 up to 1:6. The complex impedance of all
geometries is measured in different bias configurations to study the evolution
of the small-signal limit superconducting transition parameters, as well as the
excess noise. We show that high aspect ratio devices with properly tuned
critical temperatures (around 90 mK) can achieve excellent energy resolution,
with an array average of 2.03 +- 0.17 eV at 5.9 keV and a best achieved
resolution of 1.63 +- 0.17 eV. This demonstrates that AC-biased TESs can
achieve a very competitive performance compared to DC-biased TESs. The results
have motivated a push to even more extreme device geometries currently in
development.
