TabPanels have height and width properties. Assign them the numeric values with px
<ajaxtoolkit:tabcontainer runat="server" xmlns:ajaxtoolkit="#unknown">
OnClientActiveTabChanged="ClientFunction"
Height="150px"
Width="400px"
ActiveTabIndex="1"
OnDemand="true"
AutoPostBack="false"
TabStripPlacement="Top"
CssClass="ajax__tab_xp"
ScrollBars="None"
UseVerticalStripPlacement="true"
VerticalStripWidth="120px"
>
<ajaxtoolkit:tabpanel runat="server">
HeaderText="Signature and Bio"
Enabled="true"
ScrollBars="Auto"
OnDemandMode="Once"
<contenttemplate>
...
</contenttemplate>
/>
</ajaxtoolkit:tabpanel></ajaxtoolkit:tabcontainer>