Visual acuity and offset stimuli I'm currently setting up and experiment that utilises a visual search task that contains a circular array of target letters and a distractor that falls outside the circle.

Obviously the further away from the fovea the distractor moves the more difficult it is to see.

Can anyone point me to some literature as to how one would go about scaling the size of the distractor relative to the target letters to maintain comparable levels of acuity.

Most of the literature I've read doing similar experiments make the distractor bigger for this very reason. |But I'm having a hard time figuring how how one goes about deciding how much bigger the distractor should be.
Visual acuity is highest in the foveal region, namely around 1/60 of a degree, or 1 minute of arc (1 MAR). At about 30 degrees of eccentricity, visual acuity is reduced to anywhere between 1.5 and 10 MAR when determined by a shape recognition task, as shown in Fig. 1 taken from Lie, 1980:



A later study (Anderson et al., 1991) shows the following picture:



This figure shows that chromatic (color) visual acuity is about 60 cycles/degree in the fovea (i.e., 1 MAR) and approximately 0.4 cycles/degree at 30 degrees eccentricity temporally, which means a reduction in acuity of 150x.  At 30 degrees eccentricity to the nasal side acuity drops to 1 cycle per degree, which means a 60x reduction. Under achromatic (black-and-white) conditions it drops about 60x temporally and 30x nasally at 50 degrees eccentricity. These tests were done using colored (red/green) and black-white grating-visual acuity tasks. 

The Lie, 1980 study mentions several references and the Anderson, 1991 study is definitely worth checking out as well.

To get back to your scaling question: it is difficult to directly translate these findings into scaling factors for your stimuli as you make no mention of their approximate size, or their eccentricities. What the figures do show, however, is that people may not be able to see much detail in their periphery and that acuity may drop as much as 150x at an eccentricity of 30 degrees. It also shows that black-and-white stimuli may be better visible in the far periphery and also, quite interestingly, that acuities deteriorate at a faster rate with eccentricity in the temporal retina.