Bsrhrki

I tried that middle, ,middle|, and mathrelBig. I read How to automatically scale `mid` within delimiters and

usepackagemleftright
usepackageamsmath

begindocument

beginequation
MASE = frac1n mathrelBig sum_i = 1^n hatSigma^-1/2beginpmatrix
y_ci - haty_ci\
y_ri - haty_ri
endpmatrix mathrelBig,
labeleq:mase
endequation

enddocument

I believe you want the absolute value:

documentclassarticle
usepackageamsmath

begindocument

beginequation
mathitMASE = frac1n
left|
 sum_i = 1^n hatSigma^-1/2
 beginpmatrix
 y_ci - haty_ci\
 y_ri - haty_ri
 endpmatrix
right|,
labeleq:mase
endequation

enddocument

I've often seen mid misused as vertical bar; it indeed typesets a vertical bar, but considered as a binary relation.

For the absolute value you should use lvert and rvert, which can be abbreviated as left| and right| when automatic sizing is desired.

Here's a solution that employs the macro DeclarePairedDelimiter (provided by the mathtools package) to generate absolute-value "fences" around the summation. I would use abs[bigg] rather than abs*, to keep the vertical bars from becoming needlessly large. I would also write widehatSigma rather than hatSigma, to make the "hat" symbol a bit easier to spot. And, do write either mathrmMASE or mathitMASE, to inform TeX that it's dealing with an entire word rather than a product of the variables named M, A, S, and E.

documentclassarticle
usepackagemathtools
DeclarePairedDelimiterabslvertrvert

begindocument

beginequationlabeleq:mase
mathrmMASE = frac1n 
abs[bigg] sum_i = 1^n widehatSigma^-1/2
beginpmatrix
 y_ci - haty_ci\
 y_ri - haty_ri
endpmatrix 
endequation

enddocument