Abbreviation list align left with space between

The easiest way to me seems to be to load enumitem and define a new list:

\usepackage{enumitem}
\newlength\myitemwidth

\setlength\myitemwidth{5em} % <<< choose what you need here
\newlist{myacronymlist}{description}{1}
\setlist[myacronymlist]{
  labelindent = 0pt ,
  labelsep    = 0pt ,
  leftmargin  = \myitemwidth ,
  labelwidth  = \myitemwidth ,
  itemindent  = 0pt ,
  format      = \normalfont
}

and then tell acro to use this list:

\usepackage{acro}
\DeclareAcroListStyle{myliststyle}{list}{
  list = myacronymlist
}
\acsetup{ list-style = myliststyle }

Versions prior to v2.2 use the deprecated option list-type:

\usepackage{acro}
\acsetup{ list-type = myacronymlist }

The complete example:

\documentclass{article}

\usepackage{enumitem}
\newlength\myitemwidth

\setlength\myitemwidth{5em} % <<< choose what you need here
\newlist{myacronymlist}{description}{1}
\setlist[myacronymlist]{
  labelindent = 0pt ,
  labelsep    = 0pt ,
  leftmargin  = \myitemwidth ,
  labelwidth  = \myitemwidth ,
  itemindent  = 0pt ,
  format      = \normalfont
}

\usepackage{acro}
\DeclareAcroListStyle{myliststyle}{list}{
  list = myacronymlist
}
\acsetup{ list-style = myliststyle }

\DeclareAcronym{loT}{
  short = IoT ,
  long  = Internet of Things ,
  class = abbrev
}

\DeclareAcronym{dbms}{
  short = DBMS ,
  long  = Database Management System ,
  class = abbrev
}

\begin{document}

\acuseall
\printacronyms[include-classes=abbrev,name=Abbreviations]

\end{document}

---

For version 3 of acro the fastest adaption of the above code would be

\usepackage[version=3]{acro}

\SetupAcroTemplate[list]{description}{%
  \let\description\myacronymlist
  \let\enddescription\endmyacronymlist
}

otherwise using the same definitions as above. This changes the default list template. One could define a custom template which does the same:

\usepackage[version=3]{acro}

\NewAcroTemplate[list]{custom}{%
  \let\description\myacronymlist
  \let\enddescription\endmyacronymlist
  \UseAcroTemplate[list]{description}[0]%
}

\acsetup{list/template=custom}

In order to run without warnings with version 3, class should be replaced by tag in the acronym definitions and include-classes should become include in the list setup:

\documentclass{article}

\usepackage{enumitem}
\newlength\myitemwidth

\setlength\myitemwidth{5em} % <<< choose what you need here
\newlist{myacronymlist}{description}{1}
\setlist[myacronymlist]{
  labelindent = 0pt ,
  labelsep    = 0pt ,
  leftmargin  = \myitemwidth ,
  labelwidth  = \myitemwidth ,
  itemindent  = 0pt ,
  format      = \normalfont
}

\usepackage[version=3]{acro}

\SetupAcroTemplate[list]{description}{%
  \let\description\myacronymlist
  \let\enddescription\endmyacronymlist
}

\DeclareAcronym{loT}{
  short = IoT ,
  long  = Internet of Things ,
  tag   = abbrev
}

\DeclareAcronym{dbms}{
  short = DBMS ,
  long  = Database Management System ,
  tag   = abbrev
}

\begin{document}

\acuseall
\printacronyms[include=abbrev,name=Abbreviations]

\end{document}

I introduce the leftitemize environment, that places its labels in a 2cm wide left-aligned box (in bold). The labels are aligned with the enclosing environment margin.

\documentclass{article}
\usepackage{enumitem}
\let\svitem\item%
\def\mybox#1{\makebox[2cm][l]{\bfseries#1}}
\newenvironment{leftitemize}
{\renewcommand\item[1][$\bullet$]{\svitem[\mybox{##1}]}%
  \begin{itemize}[leftmargin=\dimexpr2cm+\labelsep]}{\end{itemize}}
\begin{document}
\noindent Here is my left margin
\begin{leftitemize}
\item[FFT] Fast Fourier Transform
\item[GPRS] General Packet Radio Services xxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxx
\end{leftitemize}
\end{document}