The usual quick-and-dirty workaround when ItemSize does not do what is expected is to wrap contents into an invisible frame. One can use Framed with FrameStyle->None to hide the frame or Pane. In both cases, a magic number is required to convert between line-height of Grid and printers points of Framed/Pane - it is highly platform-specific (see here). The simplified example with Pane:
dot = Graphics[{Pink, Disk[]}, ImageSize -> 40];
magic = 10;
tab = {
{"Type", "Name", \[SpanFromLeft], \[SpanFromLeft], dot,
"Esculap", \[SpanFromLeft], \[SpanFromLeft]},
{\[SpanFromAbove], "Dia.",
"Spec", \[SpanFromLeft], \[SpanFromAbove], 8.3 "mm",
Pane[11, {Automatic, 2.7*magic}, Alignment -> Center], "6.3"},
{dot, "Flycatcher", \[SpanFromLeft], \[SpanFromLeft], dot,
"Apus", \[SpanFromLeft], \[SpanFromLeft]},
{\[SpanFromAbove], 6.9 "mm", 17, "9.8", \[SpanFromAbove], 7.8 "mm",
Item[8, ItemSize -> {Automatic, 2.7}], "6.7"}
};
Grid[tab, Spacings -> {Automatic, {{0, -1.1}}},
Alignment -> {Center, Center}, Dividers -> All]
One can get rid of the overhang (gap with missing dividers) by wrapping Item around the cell:
Item[Pane[11, {Automatic, 2.7*magic}, Alignment -> Center]]
This issue of gaps is only present in version 8. My guess is that as ContentPadding -> True was added to Pane in version 8 only, in version 7 a different behaviour is the default for padding inside Pane that does not introduce these gaps. However, calling the above example with ContentPadding -> False in v8 does not remove the gaps contradily to my expectations.
An almost identical result can be obtained if given cell is wrapped in Framed (note the horizontal increase in size of the cell):
Framed[11, ImageSize -> {Automatic, 2.7*magic}, FrameStyle -> None]
